The Demon Portal is a block added by Blood Magic that controls an invasion of demons. The Demon Portal is created by sacrificing a demon at a Blood Altar placed on the Master Ritual Stone of an activated Convocation of the Damned.
The Demon Invasion
An activated Demon Portal will start spawning in a network of cobblestone roads and buildings. This city will grow to about 1000x1000 blocks eventually. Many of the buildings and structures spawn with Demon Chests containing dungeon loot, and Demon Grunt and Demon Grunt Guardian mobs will spawn in the streets. Being demons, these mobs spawn with 200 health (100 hearts).