User blog comment:CopperCookie/Apocalypse Mod: NPCs/@comment-9118156-20131021202026/@comment-4692985-20131022130414

Ah yes, I have not made a real article about it yet.

It's a thing that happens in both world after the Bringer (main boss) has been destroyed. It's a bit like a blood moon. Many monsters that can be attached to nightmares (worms, skeletons, reapers, ghosts, zombies) spawn, and they are mostly tougher versions of already existing monsters. The mobs have about 800-1000 health each and do around 60-100 damage, so yeah, pretty menacing.

NPCs also sell new things at a Nightmare, much like the Blood Moon.