Slag

Slag is the leader of the Forged, a fanatical gang of fire-worshiping raiders established in the Saugus Ironworks in Fallout 4.

Background
Slag is the leader of the Forged and is originally from outside the Commonwealth. 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, or convinced them to pass the trials. Eventually, negotiations were reached, ending the conflict, as well as inspiring more people to join, and prompting the Forged to remain in the region. By the time the Sole Survivor reaches the Ironworks, he has a strong, well-equipped raider army. However, scrap iron shipments had stopped, limiting the effectiveness of the facility, prompting him to send Bedlam to Dunwich Borers to obtain more.

He wields Abraham Finch's Shishkebab sword, given to him by Jake Finch as a bribe to gain his favor in order to join the Forged.

Quests

 * Out of the Fire: Slag and his gang are in the middle of the initiation ritual of Jake Finch when the Sole Survivor enters Saugus Ironworks's blast furnace. Whether the Sole Survivor decides to save Jake or not, Slag will be hostile after a short dialogue.
 * Here There Be Monsters: If Slag has not been killed previously, he will be leading the initiation ritual of Jake Finch in the blast furnace and the Sole Survivor will have to kill all the Forged to recover the dampening coil. If Out of the Fire has not been started, then Slag will be known as "Forge leader."

Appearances
Slag appears only in Fallout 4.

Behind the scenes
Slag is a byproduct from the process of smelting to separate a metal from its ore.