Revenants

Revenants are a moderate to high-level form of corporeal undead, relatively uncommon — uncommon enough that only Ayduin and Bodaway had heard of them prior to their time in Barovia.
The reason for revenants' rarity is fairly obvious: a revenant is formed only when a mortal soul with a powerful enough will dies in a cruel and unjust way, and is dedicated enough to revenge that they drag themself back from the beyond, initially reclaiming its original body, and claiming new ones as necessary until its goal is met or its time is up. Normally, a revenant only has a maximum of one year to enact their revenge. While pursuing vengeance, they are given the ability to always track any target they've sworn an oath of vengeance against, even across planes. They are very difficult to kill, being hardy; resistant to many forms of damage; immune to poison, charm, exhaustion, fear, paralysis, and stun effects; and capable of regeneration unless they take radiant or fire damage.
Barovia was home to a particularly long-lived breed of revenants, formed out of many of the knights of the Order of the Silver Dragon, led by and bound to the will of Sir Vladimir Horngaard. By the party's understanding, the revenants of the order maintained their undead state for nearly four centuries, before finally being laid to rest by the lighting of the beacon at Argynvostholt, allowing the spirit of their former leader Argynvost to help them let go of their need for eternal revenge.
The only remaining revenant of Argynvostholt is Sir Godfrey Gwilym. How long he will maintain his semblance of life past the destruction of Strahd von Zarovich is unclear.