The Prophet of Regret in Halo 2. For context, I'd gotten into Halo Wars (an RTS) well before any of the FPS Halo games, and I was a Regret main. The Prophet of Regret hero unit could get really, really powerful, and I was looking forward to getting to fight the force of devastation I used so often in Wars, since most other enemies I'd fought mapped fairly well to their RTS incarnations. Regret... not so much. He's mechanically treated as a vehicle, which means you can instakill him by meleeing his throne, and he's invulnerable to actual ranged attacks, requiring you to use the cheesy-feeling melee 1HKO to kill him.