If Players Die

Show players: Congratulations, You're Dead

Make a brand new character and introduce them to the campaign in the usual way
Let them use their same character sheet under a different name
- {maybe their soul was transferred like others in Barovia}
- Give them some sort of flaw
Let them play an NPC
- Ireena Kolyana will follow the group for a long time
Make them a ghost, and give the party a chance to revive them

Resurrection

Resurrection is possible in Barovia by the The Abbot in Krezk.
- Native Barovians drop hints that the Abbott is a power man of The Morninglord known for creating miracles
- There is a rumor he brought back a young boy from the grave
Much more secretly, Jeny Greenteeth and Madam Eva can resurrect the dead
- Neither will do so lightly

The Ghost Effect

After a character dies, have them come back as a ghost (not an actual ghost monster, btw). After all, their soul can’t leave Barovia because of the mists, so even canonically it makes sense.
As a ghost, the PC is invisible and can't interact with the living world in any way. The other players can't see or hear the dead PC at all

Possession

The ghost player has the ability to temporarily possess other intelligent creatures (must have the aptitude for language)
- so that they can continue to interact with the campaign
- When trying to possess a creature, they must enter a charisma contest with the possessed
- On a success, the dead PC gains full control of the creature's body
- They keep their own mental stats and abilities
- they must use the physical stats of the possessed creature
Possession lasts up to 3 hours, at which point the PC ghost is expelled from the body
- They must take a short rest before they can attempt possession again
- If the PC attempts to possess the same creature as before, the creature has advantage on their charisma contest to fend off possession

The Mist Wall

As a ghost, the PC is still technically attached to their body, even if they possess another creature, and cannot go farther than 300 ft from their corpse
At that radius, the dead PC can see an obtrusive wall of mist 360 degrees around them
If the ghost PC approaches the mist, they can see ominous, gargantuan dark shapes moving within (Dark Powers)
- as well as see the distant shapes of other dead humanoids running in the mists (dead Barovians waiting to be reincarnated and trying to hide from the prowling Dark Powers)
Every day the PC remains dead, the radius of the mist wall shrinks by 30 ft.
While a ghost, a PC has 1d8+2 days to be resurrected into their own body. After that, their body becomes too decayed to properly sustain life
The PC has 10 days (until the mist wall shrinks completely) to be resurrected at all, possibly into another body
- This can be a constructed creature without a soul, like Vasilka, or a soulless Barovian
- The Abbot, Jeny Greenteeth, and Madam Eva are the only ones capable of tying a soul to a new vessel
- Once the mist wall closes and consumes the PC ghost, they are officially and irrevocably dead and have to roll a new character.

A Player Character as Ireena (source)

If at all possible, make Ireena a Player Character. Now, this doesn't mean you have to force Ireena's personality down one of your player's throats. Instead, focus on the function of Ireena's character.
Ireena is the object of Strahd's desires; a reincarnation of his lost love that he can never obtain. That is literally Ireena's only function in the narrative of this campaign. Everything else about her is just fluff and character development.
If you've got a player that wants to play a human female, give her this function. Here's how to adapt her for the role:
- Her Name: I'm hoping this is common sense, but I'll say it anyway. Don't force your player to be named Ireena. Their character is quite literally replacing Ireena, so the name doesn't matter.
- Her Appearance: It doesn't matter how the player wants their character to look. Don't force her character to look like Ireena. Instead, Tatyana and all her other incarnations look like the PC.
- Her Personality: Ireena's personality as an NPC is already up for interpretation. So however your PC wants to portray her should be fine.
Her Backstory
- If the PC grew up somewhere other than Barovia, maybe a Vistana smuggled her out of Barovia as a baby and she has no idea. Maybe her birth name is in fact Ireena, but she just doesn't know it.
- If she comes with a pretty homey backstory, try to work with your player so that her character is a bit of a shut in. She's stayed pretty locked up in her house in the Village of Barovia and so doesn't know very much about the rest of the dimension. Beyond the simple names of towns, she's quite ignorant. When the evil king Strahd came to her doorstep, she fled into the Svalich woods.
- If she's from Barovia, she gets lost in the woods and stumbles on the rest of the party as the mist draws them in. They then go to Death House together.
Overall, having a PC take the role of Ireena in the campaign will put a lot more shock value on the narrative. Strahd's pursuit won't just be for a random NPC, but for another player. And players are way more important in dnd and automatically built a faster relationships with the other players.