Slag

Slag is the current leader of the Forged, an especially fanatical gang of fire-worshipping Raiders 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 including several former Gunners. By the time the Sole Survivor reaches the Ironworks, he has a very strong raider army. He currently wields Abraham Finch's Shishkebab sword which Jake Finch gave to Slag to gain his favor and attempt to join the Forged.

Appearances
Slag appears only in Fallout 4.

Behind the Scenes
In real life, the wasted metal after a welding operation is called slag.

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