Final Fantasy X - Nemesis

Nemesis is a secret fiend in Final Fantasy X that is created by the old man who runs the Monster Arena in the Calm Lands. Nemesis looks similar to Omega Weapon, and Ultima Weapon. Often nicknamed Omega Gold, it is the most durable and most challenging enemy in the game (with the exception of Penance in the PAL and International versions of the game), with nearly 10,000,000 Hit Points and an array of spells and attacks that can instantly kill even the highest-level, best-equipped parties.