Megalovania

The word is a synonym for Megalomania and its meaning, more so the anti-Megalomania, see Vendetta, with lawful intent of avenging friends and/or family, with more intensive power to return.

Gathering from this, we can assume that a Megalovaniac is someone with focused power avenging someone, rather than killing, and gaining power.

He may have been greatly powerful, but he didn't count on him having such a powerful Megalovania, overpowering him entirely.