Abaddon

Abaddon is the name of a dark fallen god that was outcasted by the Gods of Tyria long ago and exiled to the Realm of Torment.

According to the lore, the Margonites worship Abaddon and are his loyal servants:


 * "Civilized nations recognise revere the Five True Gods. But heretics speak of a sixth god...a fallen god. A thousand years ago, the spiritual ancestors of these heretics, the Margonites, fought an epic battle on the northern plains of Elona. The resulting carnage created a vast wasteland - the realm now known as the Crystal Desert.
 * Empowered by the blessings of their dark deity, the Margonites waged wars against the followers of the Five Gods - smashing temples, desecrating shrines and butchering all rivals. Yet, despite the awesome power granted by horriffic transformation, their army was annihilated; their false god was exiled to a realm of torment.
 * Now the wise see signs that this fallen god may be at work in the world again. His name was eradicated from all history, burnished from all monuments, condemned as blasphemy...yet still, the servants of the unholy watch and wait. With sharpened blades and practiced spells, his followers prepare for the return of their forgotten and forsaken deity."

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). In Greek, he has the name Apollyon. For more information see the Wikipedia article about Abaddon.