Demon Portal

Demon portals are structures that can spawn in a Region. They randomly cause some friendly and enemy NPCs to become possessed. Once possessed, an NPC becomes tougher and will always respawn as the possessed variant until the demon portal is destroyed. Destroying a demon portal causes all possessed NPCs to become normal again. Possessed NPCs can have some good drops, so leaving a demon portal up might be a good idea, unless an important friendly NPC, such as a Flightmaster or vendor is possessed.

Some people have reported that Demon Portals can come in varying sizes.

Destroying a Portal
Destroying a demon portal is easier than clearing a dungeon. There will be some Dark Cultists and Imps around the portal, the former using a beam on the portal. Attacking the portal will cause more imps to be spawned periodically until the portal is destroyed. In terms of difficulty, destroying a portal is easier than clearing a dungeon.