Xebenkeck!

First I need to find all the books ... I admit, I've never had to have the patience to deal with it.

Nobody's perfect. Yes, true, and, this is only five books... now you can imagine, how much I love the gathering quests. These are the real nightmares for me.

Who's Xebenkeck?

Huh. Maybe one of these days I should actually find all the books. I usually get one or two and then totally forget about it.

Never tried on Nightmare, nor will I ever, I think. Beating him on Normal was hard enough, thanks. XD

It's a significant part of Act 2 with about 4 challenging boss fights , very hard to pull out without cowardly kiting and needing effective use of combo mechanics. You all are missing a lot if you don't go through quest.

I suppose it's possible to activate the tome fast enough that it appears all the demons are from the tome. However, that's not the case. The demon horde consisting of arcane horrors, desire demons, a revenant, and more rage demons and shades than I can count, are the nexus guardians. There are around 3 waves, the first one appearing once you enter the chamber with the golem. Once they are all defeated, the golem says the gauntlet has been passed. After that, there's still the tome to deal with.

Oh yes. Sorry about that. That prior fight officially does not count as fight for the Tom, but it's hell on nightmare. Last time I beat it without running out to the corridor. .

I am not sure you can activate the tom before killing the guardians.

Actually it's 5 waves, and until normal difficulty there is pause after wave 3. See wave information here. dragonage.wikia.com/wiki/Abandoned_Thaig#Strategy

Ignore the kiting strategy. . More often than not the Arache horror will one shot your entire party hiding in the corridor if you don't kill enemies fast enough. And it's cheesing.

2 strategies worked for me.

1. Get some fire resistance ( optionally spirit if you can for boss level abomination and desire demon). Kill the minion shades very fast and than rage demon fast enough.
2. Pile up cold damage, immediately kill the rage demon. In this case next wave would appear immediately after you killed first rage demon), send your tank to desire demon, while rest of the party kill shades and abominations.

And in either case.
1. Rage demons and the arcane horror are priority.
2. Offence is best defence yet you need to have good defense.
3. Shield Anders ( you will need his haste). Have your rouge strategy set to Back to back when anders is attacked.
4. Do not stay in a place except for your warrior who should be taunting and engaging desire demon and revenent. Avoid center of the hall.
5. Hit and run works here. Actually I found out concentrate fire on one enemy can quickly overwhelm you non tanks.
6. Rouge with decoy is very helpful.
7. Do not let rage demon live too long. They will steal your potions.
8. Have anders switch between Cold ( as long as rage demon is alive) and electricity staff ( for everything else). He should have elemental weapon and heroic aura on.
9. Save Haste and winters grasp ( careful about the friendly fire) for Rage demon and Arcane horror. Definitely save petrify, horror for Arcane horror.
10. Absolutely not let the Arcane horror ( who will appear on wave 5), wiggle it's hand or teleport. He can one shot even Aveline.
I save before the fight and experimented with different party composition. Easiest was with
Fenris DPS warrior , my shadow-assasine rouge, Anders, merril or Varric. I actually did it without anders ( rouge , Fenris, Varric and merril) but needed a lots of potions including rock armor potion. Even Sebastian works fine here. His guardian angel talent is lifesaver.

None of this worked in actual Xebenceck fight. The revenent asshole appears too soon.