i couldn't kill Iytachu Xvim so i killed the 10 slaves so the Radiant pool is still activated. Now when I fight them in the first room, the monks pop in after a little bit and start attacking the twins. Now I've noticed that the monks will sometimes hit with like a 28 or something, but no damage will pop up. But when my characters try to hit they need a 35 or more to get through. Occasionally I can reduce Madae to half hit points causing them to go and get healed. then thats when the trouble starts. I can hurt them to about "barely injured" and then all of a sudden they're "uninjured" a few seconds later even though they don't cast any spells or anything.

Also does anyone know what spells seem to work best against them? I've found chain lighting works sometimes, acid storm does, tremor works wonders, destruciton and harm sometimes work, cone of cold works pretty good, but most others don't.

First of all, Jarlaxle, welcome to the boards! Secondly, without deactivating the Pool of Radiance, you will have one hell of a fight on your hands. Simple because each and everytime they even split a fingernail, they will use the pool to heal themselves. Now, since they are half-demons, spells are going to be tough to get through. If I remember correctly, fire does nothing, electricity should do nothing, but if you score with chain lightning, then way to go! Also, cold and acid only does half damage.

Furthermore, the avatar of Xvim is defeatable...if you are of sufficient level. Personally, I think that it's worth your while to beat him, as I would rather have the extra demons to defeat then the Pool still active. Since they are demons, and they are evil, I'd use spells that have extra damage to chaotic evil opponents. Same with weapons. I find that Holy Word works wonders!

Also, if you have any protection from magic scrolls, use them on the twins. Why, you ask? Because they won't be able to cast their very many deadly spells. My personal favorite is to cast it on Madae, and attack the hell out of Isair. So, she won't be able to heal. Well, I don't know what else to suggest. Good luck!

elecrizity damage doesn't do really a lot in the privat chamber of isair and madae, but as soon as they flee spells like chain lightning work wonders. the problem is, that isair can often evade the bolts. the best spell to hurt them that i found so far is horrid wilthing, because it does a hell lot of damage and their magic damage immunity isn't that high.

the easiest way to defeat xvim is to animate some undead and leat them hit him. then you should let your spellcasters attack him with elecricity and magic damage spells and especially tremor. to that one he is quite vulnerable. then if he is unconcious everything hits better anyways. one of your fighters should take "cloudkiss" ("stormshifter or s.th. like that in HoF) since it does electric damage.
oh and btw - get your peoples away when he casts sunfire or meteor swarm or funny stuff like that.

I finally took out Isair and Madae. First it seemed impossible, but then I tried that prot. from magic on both twins and after that it was ridiculously easy. Lucky I had those scrolls.
Also remember to kill that De Naly fellow, because he is the one behind those summoned monsters.

I was about to try that Heart of Fury mode, but I couldn't find it anywhere. I thought I have seen it before but now it was never to be found. Where could it be found, and are there any decent mods that I should install to make another round a little different?

You find Heart of Fury in the configuration settings for the game.

You need +5 weapons to hit Isair and Madae.
They have a damage resistance of 30 for any weapon up to +4.

So if you have a bunch of +5 weapons, equip them even if you're not proficient with them. The fight should become rather easy then (although I've never battled them without purifying the pool first...).

I find that Isair and Madae are quite easy as long as you layer your party with the right protections. Of note- Holy Aura, Stoneskin and Iron Skins, Mass Haste, Emotion:Hope, Seven Eyes, Aegis (if you want), Exaltation, Chant, and most of all, Magic Circle against Evil!!! With the last spell, you won't need to worry about demons, because they can't hit you. In this way, purifying the pool is the best way to go.