Phantom knights are the beefiest, scariest, shield-totin'est undead monsters in the game.  They spawn in dungeons or during "sinister force" time, and are a force to be reckoned with. Fortunately any found outside a dungeon will wander randomly and will almost never try to attack the player unless you attack or approach them first.

They will reduce incoming damage with their wielded shield; therefore, strong attacks and magic are recommended.
Phantom Knight
Phantom Knight2