No personal experience here, just posting based on trustworthy posts and .dats. Post your experiences here.
Rewards
Mark of the Einherjar (Permanent key item): Obtained after clearing your ninth chamber. It used to enter any chamber regardless of feathers.
1000 ampoules
Dendainsonne: Behemoth horn x1, some other behemoth material probably x1
Stoorworm: Hydra meat x1, some other hydra material probably x1
Freke: Cerberus meat x1, Cerberus claw x1
Gorgimera: Khimaira mane x1, khimaira horn x1
Vampyr Jarl: Dragon blood x1, some other blood item probably x1
Motsognir: Demon horn x1?, Demon skull x1?
Abjurations: Some head; if anyone minds translationg: ??????; H. legs
(Note: It seems you can get 1-2 material drops here. The first material is 100%, while the other is not.)
Wave Mechanics
Each wing has a different way of adding a new wave of mobs. I'm not sure if there is any set pattern here, so far I think we have 2 mob groups -> 1 group -> 1 group + boss, 1 group -> 2 groups -> 1 group + boss, 1 group -> 1 group -> 1 group + 1 boss, 2 groups -> 1 boss (+boss's "pets"). It seems that # of mob groups can vary from 3-4 + 1 boss, and waves can vary from 2-3.
Bosses
Dendainsonne: Behemoth
-Has weird "emotes":
Fast cast MeteorHeh heh heh...?Prompt?
?Possible Special Code: 1F??Possible Special Code: 0F?Debug: Dendainsonne's Rage = 0.?Prompt?
?Possible Special Code: 1F??Possible Special Code: 0F?Debug: Dendainsonne's Rage = 1.?Prompt?
?Possible Special Code: 1F??Possible Special Code: 0F?Debug: Dendainsonne's Rage = 2.?Prompt?
?Possible Special Code: 1F??Possible Special Code: 0F?Dendainsonne looks angry...?Prompt?
?Possible Special Code: 1F??Possible Special Code: 0F?Dendainsonne is enraged!?Prompt?
?Possible Special Code: 1F??Possible Special Code: 0F?Dendainsonne is out of control!?Prompt?
?Possible Special Code: 1F??Possible Special Code: 0F?Dendainsonne appears to be mocking you...?Prompt?
Kickout (higher damage than normal KB)
Thunderbolt
Shockwave
Draw-In (he tends to draw you in to his rear to trigger kickout)
When he is "enraged" he casts an instant Meteor.
Stoorworm: Hydra
Freke: Cerberus
Gorgimera: Khimaira
Vampyr Jarl: Vampyr
-"Transforms" into Vampyr Bats/Wolves? (6 of each)
Motsognir: Dvergr
-Has 12 Demon NMs (see: http://en.wikipedia.org/wiki/Arngrim#Arngrim.27s_sons) (4 WAR + 4 BLM + 4 DRK?) that prevent you from damaging him. After each one is defeated, he loses about 4-5% of HP, which will leave you him with about 40-52% health left after all Demons are killed. The Demons are:
Angantyr - WAR
Barri - WAR
Brami - DRK (Absorb-VIT)
Bui - BLM (Blaze Spikes)
Hadding the Elder - BLM (Blaze Spikes)
Hadding the Younger - DRK (Guessing only BLM/DRK use scythes)
Hervarth - WAR
Hjorvarth - BLM (Sleepga II)
Hrani - DRK (Thunder II)
Reifnir - BLM (Blaze Spikes)
Tind - DRK (Absorb-AGI)
Tyrfing - WAR (Aspir did 0)
Confirmed mobs
Berserkr x12 -- Kindred-type Demons (6 BLM + 6 DRK)
Margygr x12 -- Ahriman
Odin's Jester x12 -- Devilet-skinned imps
Corrupt Einherjar x12 -- Tauri
Audhumbla x6 -- Buffalo
Marid-X x6 -- Marids
Wivre-X x6 -- Wivres
Experimental Poroggo x12 -- Poroggos
Liquified Einherjar x12 -- Flan
Soulflayer x12 -- Soulflayers
Manticore-X x6 -- Manticores
Ormr x6 -- Shadow dragons
Logi x12 -- Bombs (magic casting; growing)
Flames of Muspelheim x12 -- Clusters
Infected Wamoura x6 -- Wamoura
Utgarth Bat x12 -- Bat solos (Red)
Utgarth Bats x12 -- Bat trios (Red)
Utgarth Leech x12 -- Leeches (Blue)
Djigga x12 -- Chigoes (Blue)
Einherjar Eater x12 -- Worms (Black)
Hafgygr x6 -- Scorpions
Vanquished Einherjar x12 -- Draugar (6 BLM + 6 DRK)
Vampyr Dog x12 -- Hounds
Undiscovered mobs
The following are in the .dats, but have yet to be reported anywhere, and since there is no mobs other than Odin and Fomors in Valgrind, it's safe to assume they belong here.
Idun x12 -- ?