Slag

Slag is the current leader of the Forged, a group of fire-worshipping fanatics established in the Saugus Ironworks.

Background
Slag is not from The Commonwealth and he presumably may have been part of a raider gang. When first accepting recruits, it was a slow process, as many of the recruits died early in the trials. At some point he engaged in conflict with the Gunners and killed many notable members, which inspired more people to join. By the time the Sole Survivor reaches the Ironworks, he has a very strong raider army.