I had done a search before I posted my question! Not only here but on google too. I wouldn't pester you people if I could find help elsewhere. (And I wouldn't start a new topic after a thread on the same was locked unless I was really desperate).
The trouble is that Mind Flayers are different from the monsters I'm having trouble with. As I said, they are somewhat vulnerable to magic and they can be killed by Couldkill or Disintegrate. But not the Ulitharid! I tried everything written on the tactics page, but they DO charm my summoned skeletons (!) and the guys still drop dead from some spell which I can't identify, no matter how closely I look at the combat log
Ulitharid attacks Keldorn. Keldorn - death. That's it.
I'll try to be more specific this time, perhaps someone can recall his own experience and tell me how he defeated these monsters.
I'm in the Athkatla Sewers, in a hidden section that was opened with a Sewer Key dropped by Tazok in Windspear Hills. There are a few Umber Hulks, Mind Flayers (which I can handle with tricks or force). The problem begins with the Ulitharid and an Alhoon. These seem to be completely magic resistant (none of my area or death spells work on them) and they have a spell or attack which I don't recognize that makes my party members drop dead instantly. Death Ward doesn't work against it.
The weird thing is that I have already been in the Illithid area in Watcher's Keep and I haven't met creatures like these even there (although I might have missed a few rooms).
The party is around level 14-15, able to cast level 7 cleric and level 6 wizard spells.
Any help would be appreciated! Thanks!