Earth-666 is an alternate reality that first appears in Marvel Comics' Secret Avengers Vol 1 #33 and appeared in the next two issues of that volume. The reason it's called Earth-666 is because in that reality, Earth is a world of the undead.
In this world there are no living humans or none are visible. The world is run by a totalitarian regime, in which "civilians" require permits for a variety of things. It's inhabitants included vampires, werewolves, ghosts, mummies, zombies, spider humanoids, fish-like humanoids, and squid-like beings. They prayed to or swore by, the Old One Shuma-Gorath and the Hell Lord Mephisto. It's superhuman defenders were the Undead Avengers.
Their god is an Undead Celestial (space god) and their objective is to spread the undead across the universe. Who possessed the Orb of Necromancy which they planned to us to help them in their goal of bringing the dead back to life throughout the cosmos. It was feared that if the rest of the cosmos found out about their agenda, they would come and destroy their planet. Located on their world was the Shuma-Gorath's Ego Maze, or at least access to it via a portal.