Lycanthropy

TLDR;

bitten -> DC 15 CON save; infected on fail
benefits:
- immune to nonmagical/non-silvered damage (wounds heal instantaneously)
- advantage on smell/hearing (werewolf) skill checks
- STR (werewolf) => 18 (+4)
- other beasts of same class regard with cautious friendship
cons:
- lose control of character at night
- cannot benefit from short rest
  - after 3 days => exhaustion
  - regain half total HP when transforming to were
at night:
- feel horrific pain shoot through their limbs and their memory goes dark
- in the morning, they wake up naked somewhere
  - maybe covered in blood
  - maybe van Richten is trying to kill them
  - maybe lost important campaign item
in battle:
- can go hulk if about to die or for good narrative reason
- transform, get half HP, must attack nearest creature (even friendly)
removal:
- Greater Restoration
  - Madam Eva, Strahd, or The Abbot (Healing Touch)
- Killing root Lycanthrope
  - Emil Toranescu
  - passed down from one elder to next
  - if he dies, all werewolves in Barovia disappear
- Belladonna Root Treatment
  - very dangerous poison that may kill PC instead of healing
  - van Richten, Kasimir Velikov, Baba Lysaga, Strahd/Rahadin, Mad Mage

Overall Clarifications and Reminders

Damage Immunities

Lycanthropes RAW are immune to nonmagical and non silvered damage. But, come on. A werewolf's skin isn't made of titanium. There's no a way a sword should just bounce off.
Instead, use some flavor and make it so that such wounds heal instantaneously. Essentially, yes, they're still immune, but this makes far more sense.
However, should an attack ever put them at negative full HP with a single hit, non magical or not, it's an instant kill. (If a werewolf's full HP is 58 and a single attack puts them at -58 I mean.) There'd be no time for the lycanthrope to heal, even instantly, from such a blow. This is just logical flavor I put in which you can take or leave as you will. :)

Inherited vs. Infected Lycanthropes in Barovia

I'll go into this in more detail in the following sections, but I'd like to lay out who's who.
Wereravens
- Currently, all wereravens in Barovia are inherited lycanthropes. The Keepers of the Feather have been very careful with their ability and don't spread it willy nilly. Unless you'd like to come up with an original NPC that's an infected wereraven instead of natural born, they're all inherited lycanthropes.
- The Martikovs are the main wereraven family in the valley. However, with mentions of NPCs like Muriel, you can assume there are one or two other families of natural born wereravens in Barovia, though perhaps not quite as large.
Werewolves
- The werewolf pack north of Kresk is made up mostly of infected werewolves, with a few inherited werewolves in leader positions.
- The following are natural born, inherited werewolves: Kiril, Emil, Zuleika, Skennis (the elder), Wensencia (the mother figure), and Franz (the book's suggested backup leader).

Drop yo shit bro

NPCs and PCs who shape change due to lycanthropy don't have their stuff change with them. Their clothes will rip and sag. Their items and packs will be dropped. Don't forget this element.

Inherited Lycanthropy

Origin

Inherited lycanthropes are more like a species than a curse. It's something that lives in a creature's DNA and is passed down from parent to child. If one parent is an inherited lycanthrope and another is humanoid, there's a 50/50 chance the offspring will be a shape changer.
- As a bit of extra lore, I've made it so that inherited lycanthropy is actually given as a blessing from great nature gods/magical beings. In CoS's case, these are the Fanes of Barovia. The Fanes are a trio of extraordinarily powerful archfey directly attached to the land who laid claim over the Valley before Strahd's arrival. Long before Strahd, the Fanes gifted certain faithful individuals with lycanthropy so that they may be closer to nature.
- While inherited lycanthropy was originally a blessing from the Fanes, infected lycanthropy is a curse. This curse was set upon those who desecrated the land or otherwise severely offended the archfey.

The Circumstances of Shape Changing

Inherited lycanthropes can change forms at will and retain their full personality and mind when in their altered states. The only time when this is not the case is on the night of a full moon, in which all inherited lycanthropes are forced to change into their were-form and loose control for an evening, following their animalistic instincts. However, even then they aren't nearly as violent as infected lycanthropes and can recognize friends and family.
Inherited lycanthropes undergo their first change at a very young age, usually around four or five years old. With enough practice and focus, they can gain full control of their change in a few short years. However, their transformed state is age appropriate. A young werewolf will change into a puppy version of a wolf while still an adolescent, for instance.
Because their change is a blessing, inherited lycanthropes experience minimal pain during their shifts.

Creating More Inherited Lycanthropes

So, what if someone marries into the family like Dankia Martikov? If they weren't a lycanthrope at first, how do they become a true part of the family without ending up an out of control monster?
- I home-brewed a sort of ritual that inherited lycanthropes can perform to create another inherited lycanthrope. Over the course of a year, on each full moon, a humanoid must accept the blood of a transformed lycanthrope, willingly given. Throughout the year, the humanoid will end up in various states of sickness and fever as a result.
- On the last full moon, upon accepting the blood the humanoid will undergo their first transformation as an inherited lycanthrope. If the humanoid misses a full moon and doesn't get the blood, their fever becomes much worse and the individual usually dies within a couple weeks.
For example:
- Upon deciding to join the family, Danika was locked in a room with Urwin during his transformation on the night of the full moon. Though he didn't retain his mind, Urwin was still able to recognize his feelings for Danika and remained mostly tame in her presence. Danika then made a cut on her arm and a cut on Urwin with a silvered dagger and pressed the wounds together, mixing blood. Danika did this every full moon for a year, going through the waves of sickness, before finally transforming.
- This is obviously a tedious and risky procedure. If the transformed lycanthrope gets spooked, they might easily lash out and accidentally kill the humanoid. If something happens and they accidentally miss a session, the humanoid will grow sick and die. Inherited lycanthropes don't usually pass on their blessing unless they are absolutely sure of the individual they've chosen.

Creating Cursed/Infected Lycanthropes

Inherited lycanthropes can also pass on cursed lycanthropy is they choose. This acts in the same mechanical vein as a Vistana's curse. With a bite attack, on a hit, an inherited lycanthrope can choose to try and curse the target, invoking the con save. This only activates if the target has done the lycanthrope a great wrong.

Infected Lycanthropy

Infected lycanthropy is much less kind than inherited. Overall, it is a curse and should be treated as such

Spreading the Curse

Infected lycanthropes both receive and spread their curse through bites and no other means.
I would recommend that you keep this simple for your sake as a DM. A cursed individual cannot birth or sire children with the same curse. Their children are normal humanoids and must be bitten themselves to become lycanthropes.

Transforming into the Were-form

Inherited lycanthropes can change into their animal form and were forms at will. Infected lycanthropes are not so lucky.
According to popular mythos, lycanthropes should change on the full moon. I would change this for the sake of dnd. Time moves quite quickly in-game and it's possible that no more than a month or two may pass during the whole campaign. I'd flip this mechanic so that an infected character changes every night except the night of the new moon. You'll get much more story value for lycanthropy that way.
- The inherited lycanthropes at the Werewolf Den keep the infected lycanthropes from running wild every night.
The only time an infected lycanthrope can change outside of the moon's cycle is during great duress, such as if they feel threatened or are quite angry.
While transformed, an infected lycanthrope completely succumbs to their beastial whims, often very aggressively. The change is exceptionally painful. While in this state, they only listen to the higher ranked inherited lycanthropes and/or Strahd.

Transforming into Animal Form

With a great deal of practice, an infected lycanthrope can change into their animal form at will. This doesn't come naturally, however. An infected individual must actively choose to give in to their beastial nature and fully embrace their curse to control this change. It takes time, effort, and quite a bit of pain.

Running a PC Infected by Lycanthropy

If a PC becomes an infected lycanthrope, RAW, they become super powerful. They're immune to all non magical damage and get Keen Senses and some kind of pack advantage. You don't want your players to think being a werewolf is a good thing. All power should come at a price, especially in CoS. Here's how I would instead run an infected PC.

Alignment

Firstly, don't change their alignment. I've always though that automatic alignment shifts were pretty shallow (I've got a oodles to say about the Amber Temple, but I'll cover that in another post). After all, it's not like most infected PCs are actively choosing to be evil when they're bitten. Alignment should be about personality and character growth, not a con save. Instead, make lycanthropy a story element.

The Benefits of Being Cursed by Lycanthropy

PCs are immune to nonmagical and non silvered damage. Wounds made by such damage heal instantaneously.
PCs get advantage on all skill checks relying on smell and hearing (if a werewolf) or sight and hearing (if a wereraven).
Their Strength score automatically is increased to 18 (if a werewolf) or their Dexterity score automatically is increased to 18 (if a wereraven). If their Strength/Dexterity is already an 18 or higher, it doesn't change.
Beasts of the same classification of the PC's lycanthropy regard the PC with cautious friendship and don't outright attack.

The Negatives of Being Cursed by Lycanthropy

Narratively, the PC changes form every night and looses control of their character. More details on this in the next section.
Because they run around each night, an infected PC can not benefit from a short or long rest unless it is taken during the day. After three consecutive days without a long rest, the PC should take a level of exhaustion, which becomes cumulative. Whenever a PC transforms into were-form, they regain half their total HP, but no spell slots or other benefits requiring a rest.

Surprise, Surprise

DON'T TELL YOUR PC WHETHER OR NOT THEY'VE BEEN TURNED INTO A LYCANTHROPE.
Have them make their save against the bite and then don't say anything. Just saying, "Whelp! You failed so this is how it's going to be now." is such a wasted opportunity.
Instead, slowly reveal the benefits and negatives of their curse. After the first night when they black out, tell them that, by the way, they don't get the long rest. If they have to roll a perception check, have them roll with advantage and don't tell them why. If they make a melee attack and their normal modifier is a +2, remember it's actually a +4 now. They'll be surprised when they start landing hits they feel like they should have missed. Make their powers a discovery and it'll be so much cooler for them.

Playing an Infected PC Narratively

Infected Lycanthropes can not control their curse or their changes. An infected PC will transform every sunset and revert to their humanoid selves every sunrise.

Each Night in Game:

Each night, the PC transforms into their wereform. However, don't describe this to them. Instead, tell them that they feel horrific pain shoot through their limbs before their memory goes dark. Only describe this change if another PC is there to witness it.
For these nights, it's much more interesting to forgo rolling for random encounters and just have the night pass without event. In the morning, the PC wakes up naked somewhere.
I'd have some fun mysteries planned out for these events. Does the PC wake up covered in blood? Who's blood is it? Is it possible they did something unspeakable? Was the PC carrying a story important item that is now lost somewhere? Is Van Richten with the party and now adamantly wants to kill the PC? This could be some really fun storytelling.

In Battle:

If an infected PC gets really excited in battle and hardcore role-plays the bloodlust OR if the PC is in dire straights with only a handful of hit points left, have them roll a DC 14 con save. On a fail, they transform into their were form, regain half their total HP and must attack the nearest living creature, even if that creature is a friend, until it's dead. If there are multiple targets within range, as the dm, assign each of them a number and have the player roll a d6. The roll will determine which target the PC attacks next.
If the PC is reduced to 0 HP while in were form, they fall unconscious and revert back to their humanoid form.

Removing the Curse of Lycanthropy

Quick Note on Inherited Lycanthropes

In the edits I've incorporated into CoS, inherited lycanthropy isn't a curse. It's pretty much a race. So you pretty much can't get rid of it. Inherited lycanthropy can only be removed by the Fanes themselves, usually because the individual has somehow wronged them or abused their blessing.

Too Simple RAW

As written, infected lycanthropes can get rid of their curse with a remove curse spell. This sounds great, except for the fact that that's a level 3 spell and easily accessible throughout Barovia. Not only can magic classed PCs get the spell pretty easily, but every priest and notable NPC seems to have the spell as well. You should make infected lycanthropy be more of an ordeal than that.
Guildsbounty's post has some great stuff on removing the curse, but I found it just a smidgen too complicated for practical use in game. A lot can go wrong and the information on the process would be hard to come by. Instead, I've borrowed and modified certain ideas as well as added a few of my own.

Better Ways to Get Rid of Lycanthropy

Greater Restoration Spell

Greater Restoration is a fifth level spell that magic classes don't even have access to until level 9. What's more, there are almost no NPCs that have the spell in game, making it much harder to find. In fact, unless I overlooked somebody (which is entirely possible, gosh there's a lot of NPCs) only Madam Eva has Greater Restoration RAW.
A couple other NPCs, like Van Richten, can theoretically switch out their 5th level slot for Greater Restoration after a long rest, but that's neither here nor there.
- In fact, from a role play standpoint, I wouldn't recommend giving Van Richten access to the spell at all. He's a monster hunter, not a monster curer, after all. Helping those cursed by monsters has never been as important as killing the monster itself in his eyes.
If you're expanding Strahd (as many DMs do), you might have given him access to a whole bunch of spells, including Greater Restoration. It'd be rather ironic if a lycanthrope player's only hope is the big bad guy himself. ;)
Lastly, I'd like to remind you guys about the Abbot's Healing Touch action, which can cure curses, disease, blindness, deafness, and poison. The Abbot is the only holy figure in game with the equivalent of a Greater Restoration Spell.

Killing the Root Lycanthrope

Almost all infected lycanthropes can trace their curse back to an inherited lycanthrope. If the root lycanthrope is killed (through unnatural means), all the lycanthropes created from its curse are also freed of their curse.
You can handle this in one of three ways to translate it into the game:
- One: The root lycanthrope of the werewolf pack and the wereraven clan long ago died of natural causes. This is no longer an option as a result.
- Two: The root lycanthrope is passed down from one elder to the next, keeping the root lycanthrope alive and ever changing in a clan. Of the wereravens, this means that Davian is probably the current root lycanthrope. Of the werewolves, Skennis or Emil are probably the current root lycanthropes. This gives PCs a viable way to get rid of the root of lycanthropy in game. However, this also opens the door to a bunch of bigger effects. If the root wolf is Skennis - old and feeble Skennis - he'd be really easy to kill and suddenly have no werewolves in Barovia. The same might be said of Davian Martikov.
- Three: The root lycanthrope for each clan lives on somewhere in the Svalich woods. This might be an ancient difficult mini-boss or an interesting NPC the players may come across.

Belladonna Root Treatment

I'm borrowing this idea straight from guildsbounty. In their post, they suggest that Belladonna (a highly poisonous real world herb) can be used as a preemptive measure to stop lycanthropy before a victim's first change. I've modified this slightly so that it can be used to sort of "kill off" the curse within any individual.

The Mechanics of consuming Belladonna Root

Takes 10 minutes to show effects
Has a bitter taste and a peculiar musky smell, making it hard to mask in foods and such.
After consumption, the target must succeed on a DC 13 Constitution saving throw or their strength score is reduced by 1d4.
After another 10 minutes, the target makes an additional DC 13 Con save. On a fail, their Strength score is reduced by 2d4 or half as much on a success.
This Strength reduction lasts until the target completes a long rest.
If Strength is reduced to 0, the target becomes paralyzed and begins dying and must roll death saving throws. Other PCs and NPCs cannot help stabilize the individual.
If the target succeeds on their death saves and is stabilized, they remain bedridden for the period of a long rest, after which they wake with 1 HP and a Strength score of 1. An additional long rest will restore both HP and Strength scores completely.
Antitoxin can stop the progression of Belladonna completely, including stabilization during the death saves. However, using Antitoxin will also completely interrupt killing off lycanthropy, leaving the curse intact.
If an infected character is given Belladonna Root while in were-from and survives the effects, they'll be rid of their lycanthropy. This method does not work on inherited lycanthropes.
Belladonna root is extremely hard to come by. However, the following individuals may either have the poison or know where to find some:
- Van Richten - If he doesn't have the root on his person, he may have some in his tower.
- Kasimir and the Dusk Elves - The elves know the woods pretty well, so it's not impossible they know where to find some Belladonna.
- Baba Lysaga - She's a creepy nature-y looking witch. She's probably got some root.
- Strahd and/or Rahadin - Strahd's got everything up in his castle. Let's face it.
- The Mad Mage - Dude's been living in the woods for years. He's got to know where to find it.