Yup. Right-click on him at different times through the battle and you will notice his fatigue going up, even when he's already unconscious and not casting any more spells.
HORNED KING SHOULDN'T ASK QUESTIONS ABOUT DEVILS !
And even if they didn't kill them via heat I don't see the point of guarding a Mound King with Devils, the MK are quite low level commanders
set devils either to hold and attack archers, attack archers or hold and attack rearmorst enemy. Guarding ANY commanders with devils is quite weird imho..