"Defeat ghost kings" is a mission objective in the quest Denial, Anger, Initiative in Tiny Tina's Assault on Dragon Keep DLC
Defeat the Ghost Kings that the Handsome Sorcerer summoned
You will be fighting:
As well as the Heads of each of them
If an Immortal Skeletaur or Eternal Skeletaur gets in the way, be sure to kill it and pull out the Sword so that it doesn't revive at a higher level.