Casadorr SzarrIf you face him in conversation, that guy will take your rouge vampire Astarion as another victim of his ritual. You will have to fight with three partymembers, if you face him with Astarion. He will surely turn into a risen vampire and kill Astarion permanently. The fight as well is very unfair, because of his strength and strong longrange AOE spells...
After some tries here comes my sollution:
Face him without Astarion (split party) and wait alone, while your main-character and the rest of the party face the Szarr-guy. Pretent to deliver Astarion to him, then he will give you "one chance" to do so. Your party may move quite freely then, make your final preparations, summon/buff etc.
Astarion sneaks in, useing his badass-poison, skills whatever to pickpocket Szarr (unnessasary - but I tried - to steal his staff - found just gold

) then I backstabbed him with a psionic-passive, poison etc. and he was down to 40%, the whole enemies where "surprised" then, I went full in on Szarr (Firewall , other AOE again XD ) the mighty Szaaaaaar was no more. The rest of the pack was killed by my AOEs and summons/party. Easy thing, noone died, some weren't even harmed.
Then calm returned to the battlefield and I healed up a little before I encountered another Szaar in his coffin. Storydriven...
Don't forget to loot Szarr (corpse at the marker!!!! NOT mandatorily where you killed him) and his coffin (Key)