Vampire

The exact properties of Exandrian vampires don't seem to be common knowledge, except for possibly their weakness to sunlight.

There is possibly some manner of connection between Vampires and Vecna, unless Silas Briarwood become and Vampire after his resurrection (which seems unlikely).

Sad Queen also seems to have possibly Vampire-like features, which could be the source of the Raven Queen's hatred of the undead.