That is a hard fight, but the Doomsayer CAN be killed. There is only one use for the idol, and you may not want to do it.
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This information is taken from the great Baldurdash site...
copied and pasted :
The second concerns another completely useless item from Baldur's Gate 1 that comes to mind as being not only worthless but almost impossible to find if one doesn't know it exists: the longsword called Vampire's Revenge. Though it sounds as though it is a powerful weapon, in fact it is cursed and wielding it causes the user to become Feebleminded. It also harms the wielder and heals its target on every strike, having earned its name by being forged by vampires to destroy adventurers. It is only obtainable from the ghost of Ulcaster, the mage founder of the Ulcaster school of magic east of Beregost, if the party happens to have the
two following items in their possession:
The idol of the lost stormlord god Kozah, who sounds like a predecessor to Talos, obtained from the quest of Charleston Nib and the tomb-diggers on the map east of Nashkel that mad Captain Bragh of the Amnish guard can be found. A difficult (for BG1 characters) opponent called a Doomsayer (rather like a blurry Battle Horror) must be defeated for the idol.
A suit of rusted armour, obtained from a Skeletal Knight in the dungeons that connect Firewine Bridge with the halfling village of Gullykin. This is the item that makes this sword nearly impossible to find without prior knowledge as it is unlikely for the party to have this in their possession: right after obtaining it, the quest is to be completed by giving the armour to a group of Ghost Knights; the skeleton was their traitorious fellow, who is reunited with them and amends made in death, and they depart with the armour.
*end copy-and-paste*
the site is at
http://members.home.net/mrkevvy/
check out the section about the Pantaloons...

I'm going to replay the whole series after TOB comes out, just to check this stuff out...
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The RudeDawg
Known in these Forgotten Realms as Perin LightEyes
and my girlfriends, Pamila and Pfil