Demonic Library is a Vicious-level dungeon located far to the North. It is unlocked by defeating Shifting Passages. The map consists of the main (library) part, as well as a special subdungeon hosting the boss of the dungeon (arena).
Succubus: Weakening Blow, Death Gaze 50%
Djinn: Magical Attack, Retaliate: Fireball
Thrall: Undead, Bloodless, Poison, Mana Burn
The boss is The Avatar ( Animated Armor icon, Attack 75, Health 954, Physical Resist 25%, Magical Resist 75%, Berserks 25%). However, in order to enter the Arena under the library where the Avatar resides, the five Demon Lords must normally be defeated:
Perivolcan Ze: Attack 105, Health 222, Magic Resist 50%, Corrosive; found in the top left corner of the map
Perivolcan Pa: Attack 105, Health 222, Physical Resist 50%, Weakening Blow; found in the top right corner of the map
Mesostel Pa: Attack 105, Health 222, Curse Bearer, Mana Burn, Deathgaze 100%; found at the right side of the map
Aphelion: Attack 105, Health 222, First Strike, Poisonous, Magical Attack; found at the very bottom of the map
Mesostel Ze: Attack 105, Health 222, Magical Attack, Retaliate: Fireball; found at the left side of the map
The Demon Lords each have identical stats, but different traits.
Defeating any Demonlord will give one layer of Death Protection to another one (chosen randomly). Defeating all five Demon Lords will remove all unbreakable walls from the map, thus unlocking the central entrance to the circular arena where the Avatar resides.
Striking the Avatar in melee or via BURNDAYRAZ produces several level 10 "???" enemies, with random palette swaps and abilities.
Because the Demon Lords have various traits, most strategies will find at least some of them more difficult to defeat. Generalist classes like the Thief or Tinker will benefit from their versatility; Paladins will have an easier time defeating the Avatar; Warlords have an easier time defeating the Demon Lords; and Bloodmages can tap into the blackspace of the Library Walls and fully benefit from the sizeable blackspace of the Arena itself.
In terms of races, because of the need to spike potentially multiple bosses, the potion races generally fare well. Orc does well because of the decent early-game damage boost that helps against the non-trivial monsters occupying the map.
Fighting the Demon Lords
Because each dying Demon Lord will give a layer of Death Protection to one of the remaining Demon Lords, leaving the most difficult lords for last might prove to be counter-productive. It is generally a good idea to start with one or two easier demon lord, but then take on the most difficult one after them, timing level-ups and using up some of the potions / other resources to defeat him.
Fighting The Avatar
Before engaging The Avatar in the bossfight, it can be beneficial to move him right next to the stairs using teleportation magic or PISORF. Otherwise the minions that spawn when he is struck will clog the whole map, making the stairs back to the surface inaccessible. Having him right next to the stairs leaves access to the resources (altars, shops, monsters, blood pools) within the library itself.
There are no quests associated with the Demonic Library, though The Avatar drops the Avatar's Codex.