[HJ]

If it was decay, he would take damage even before reaching that fatigue level. Decay has an animation, a unit flashes dark purple. Were his hitpoints down? Were there any corpse candles in the battle? Was he hit by a bane blade? Those are other effects that can cause decay other than the spell itself, and corpse candles don't have to be summoned since they can come from the lantern shield.

If it's not a big deal, you could post a screenshot or two so maybe then we'll manage to figure it out.