Doomsday is an incredibly powerful monster with regenerative abilities. Each time he dies he respawns having evolved past the last thing that killed him, making him slowly invulnerable over the course of centuries traveling the universe. Developed as a weapon in a facility by the scientist Bertron, he was tested out through many of his first deaths on the planet Krypton, giving him an immense natural hatred towards Kryptonians.
Doomsday, a villain from the Superman comics. I have always loved a titanic, hulking creature in stories. They have a visceral presence and really drive the stakes of the protagonist higher if done right.