Secret Wars #4

As the Cabal and the Thors engaged in combat, Sheriff Strange explained to the new-found survivors from the raft that when he and Doom confronted the Beyonders, they were able to kill them, and while Stephen didn't, Doom took their power for himself and used it to build Battleworld from the remains of the Multiverse. The hammer of the Thor from Higher Avalon starts glowing, meaning he was being called for battle.