It's an underground prison under the territory of Aden. Many dangerous criminals and vicious demons are held there.

Guide

Dungeon of Abyss is a hunting zone for players level 40-49. The zone is divided into the Eastern and the Western parts. Your character can teleport to the part corresponding to its level.
Talk to NPC Tores in the Town of Aden to get into the zone.
You have 60 minutes to stay in the zone to complete your quests. The mobs are not aggressive and respawn quickly which is really convenient for completing the zone quests alone.
In both Eastern and Western parts you can meet such mobs as Unworldly Overseer and Abyss Warder. They appear in one of the 7 rooms of the zone (the corresponding system message is shown). You have 30 seconds to kill them. If you don't, they will move to a different room. After they move for the 5th time, they disappear. When you destroy them, you can get the keys: a Key to the West Wing of the Dungeon of Abyss and a Key to the East Wing of the Dungeon of Abyss .
Use them to move to the area where Condemned of Infinity and Condemned of Abyss dwell to defeat them. You have 30 minutes to kill them. You can either get no reward or one of the following items:

Condemned of Infinity	Condemned of Abyss
Gemstone (D-grade) Accessory Pack (C-grade) Gemstone (C-grade)	Gemstone (D-grade) Accessory Pack (C-grade) Gemstone (C-grade) Armor Pack (C-grade) Gemstone (B-grade)

Mobs

Name	Level	EXP	SP
Wandering Dead of the Dungeon ( Wandering Dead of the Dungeon)	42	2470	75
Wandering Ghost of the Dungeon ( Wandering Ghost of the Dungeon)	42	2470	75
Wandering Evil Thoughts of the Dungeon ( Wandering Evil Thoughts of the Dungeon)	43	2589	78
Wandering Spirit of the Dungeon ( Wandering Spirit of the Dungeon)	43	2589	78
Unworldly Overseer (Wandering Unworldly Overseer)	44	2711	82
Condemned of Infinity (Captured Condemned of Infinity)	45	2835	86
Wandering Dead of Abyss ( Wandering Dead of Abyss)	46	2963	89
Wandering Ghost of Abyss ( Wandering Ghost of Abyss)	46	2963	89
Wandering Evil Thoughts of Abyss ( Wandering Evil Thoughts of Abyss)	47	3093	93
Wandering Spirit of Abyss ( Wandering Spirit of Abyss)	47	3093	93
Abyss Warder (Wandering Abyss Warder)	48	3226	97
Condemned of Abyss (Captured Condemned of Abyss)	49	3362	101

Quest	Type	Levels	Start NPC	Reward	Restrictions
Exploring the West Wing of the Dungeon of Abyss		40 - 46	Magrit	250,000 7,700 Dimensional Gift	No requirements
Exploring the East Wing of the Dungeon of Abyss		45 - 51	Iris	300,000 9,000 Dimensional Gift	No requirements