Phantom Warriors

Phantom warriors are a type of undead previously unknown to the party, seemingly unique to Argynvostholt.
Similar to a ghost in that these creatures are spectral and capable of passing to and from the Ethereal Plane, phantom warriors walk rather than fly, and are significantly more difficult to hit than a ghost. They also rely on spectral weapons rather than necrotic attacks, and have not displayed any of the abilities of life-draining or possession that ghosts possess.
Like certain types of ghosts and other trauma-induced undead, the phantom warriors of Argynvostholt seemed stuck in the past, responding to Ezmerelda d'Avenir and the party as if they were Strahd's forces, part of an assault on the stronghold many centuries ago. They did not appear capable of being reasoned with, and hit hard and fast.
To the best of the party's knowledge, these phantom warriors were a phenomenon specific to Argynvostholt, and should not be encountered again following the lighting of the beacon.

Lifespan

Eternal, theoretically