During The Alignment, Devils use their cultists to infiltrate a moon cult in Northeastern Gronua to siphon their magic during their alignment to open a gateway to Hell, allowing devils and Demons to physically enter the material plane (The Reckoning).
The Unknown were sent into the gateway while a massive army fought demons and devils on the material plane, attempting to close the portal. Many Steelborn that went through the gateway experienced the dilation of time and were corrupted by Hell, becoming the Unknown, who turned on their allies.
Lucifer sent a small army of Unknowns to close the portal, but had to fight their way through enemies on the higher levels of Hell, slowing their progress.
The Cosmic Horror and its Unknown soldiers invade Hell, coming from The Far Realm (Realmbubble).