Breathe of the Dying does have a Zod, and it has a 50% chance to cast level 20 Poison Nova when you kill an enemy, along with Prevent Monster Heal, +30 to all attributes, +60% attack speed, and -20% requirements, which is probably how he can equip it in the first place.
Prevent Monster Heal would have been nasty in that fight, and would provide quite a challenge if Xemnas would have had that ability.