Sker Ritual

The Stranger is the main boss on Sewers of the Dead. You can “kill” him in the normal zombie rounds, but this will not pop the achievement. You need to finish all the story objectives on the map which will eventually lead to a “defeat The Stranger” objective.

This battle is a little different from the other maps because The Stranger is not running around with the other zombies, but is the huge figure in the background. Shoot his purple eyes. After defeating him, this ends the “story” objectives on the map and allows you to extract from the level.