Board Thread:Speculation House/@comment-108.180.145.133-20130922140854/@comment-173.58.95.3-20130929225826

The Creatures of Grimm spawn in designated areas just out of the main character's sight, and they continue spawning until the enemy threshold has been reached, in which case the spawn spot is disabled.

Or in all honsety, Grimm comes from the Grimm Reaper's Grim fairy tales.