Abaddon

Abaddon is the name of an outcast god imprisoned by the Gods of Tyria long ago.

In the Nightfall campaign, Spearmarshal Kormir suspects that Varesh Ossa and General Kahyet, her respected mentor and loyal officer, are worshippers of this deity and are trying to bring about its return.

Abaddon has a location named after itself - Abaddon's Mouth, which symbolizes a gateway to the Underworld.

Note: It is currently unclear whether Abaddon is just an alias of Dhuum, or if these are separate beings.

Trivia
Abaddon is Hebrew and means "doom" or "destruction". In the Bible it denotes both a location (the underworld / hell), as well as a creature (a demon / satan). For more information see the Wikipedia article about Abaddon.