Death Knight

A CR level 17 monster in Dungeons and Dragons 5e
Death Knights used to be oathbreaker paladins that died without redeeming themselves.
They are in the Monster Manual

Death Knights usually command armies of undead. They are extremely powerful, throwing hellfire orbs and using their longsword. They can also parry, which is funny for the dm but fucking annoying for players

The skeletons you are fighting suddenly fight with renewed vigour. It can only mean one thing: the Death Knight has come.