Damage Immunity

Shapeshifters only gain the benefits of damage immunity when in hybrid or animal form. Humanoid form offers no resistance to damage.

Lycanthropy

Ravenloft distinguishes between afflicted and natural-born lycanthropes.

An afflicted character doesn't control their shapeshifting, changing form under the full moon and other circumstances as dictated by the DM. While so shifted, their moral alignment becomes Evil, and they'll act accordingly. The afflicted shapeshifter may or may not remember their acts of predation while changed.

A natural-born were-creature has full control over its shapeshifting.

Curing afflicted lycanthropy is also more difficult in Ravenloft. The afflicted were-creature can't be cured while the one who made it still lives. The original natural-born were-beast must be slain first and then the afflicted character must receive a remove curse spell during one of the nights of the full moon when they're transformed. Even then, the spell only allows them to make another saving throw against the lycanthropy, it doesn't automatically remove the condition. There is only one chance for a cure in Ravenloft. If the character fails this second save, they'll carry the affliction to their grave.

I Know a Guy

In the spirit of The Empire Strikes Back, when Han Solo heads to Lando for help, a player may declare that their character "knows a guy". This can happen once per story and must be dramatically appropriate.

In a situation where it makes sense, the PC declares "I know a guy" and then quickly sketches that NPC and the relationship. Fewer details are usually better - that leaves room for the DM and PC's to play.

Upon declaration, the NPC exists. Then, when the PC's try to interact with the NPC, the player who created them makes a Charisma roll to see how the NPC reacts. Depending on the situation, the DM may decide Advantage or Disadvantage applies.

This is a simple and loose mechanic inviting players to create critical NPC's and continue developing the backstory as they play.

The Rule of Cool

Failure should be dramatic and thematic but not grind play to a stop, in most cases. If a character tries to leap across a chasm, but fails the roll, they don't plummet to their death but desperately grasp at the vines jutting out from the edge. A failed Stealth check means the character makes some noise to give away their position, and now can either create a diversion or attempt to get around their target another way.

Keep things flowing, keep things fun. Player characters occupy a unique spot in a game. While failure is, by itself, part of the horror, in most cases it shouldn't be used to create an anti-climatic situation.