Selfless Knights are variants of Knights added in the Halloween Update. Their mouth were wide agape similar to a Maw's expression, and possessing Isaac's face on the back of their heads instead of a brain.
They are commonly found in Sheol, but will occasionally replace regular Knights as champion version in previous floors, where killing them will drop a Pick Up. They are very rarely found in areas all the way up to the The Caves/The Catacombs.
They behave just like regular Knights, except they advance towards Isaac at a higher speed if detected.
Eternal Selfless Knight acts exactly the same as Eternal Knight, as it can reflect Isaac's tears and leave creep upon death.