On the dark brotherhood quest whodunnit you will be told to kill everyone at summit mist manor without being seen. (it lookes like a blue troll) the monster is 100% immune to frost and has the chameleon ability, but is weak to fire kill it and take the bow from its remains - the bow has a 15 point frost damage.

Then he went back to kill the monster ( and yes, he died) after you read the journal go to the edge of the cliff and wait a couple seconds, it will say you discovered some new dungeon locations (this is optional)now go back the mountain using the same trail the monster should be running around somewhere along the trail. The journal says that the couple found the monster and it killed the wife of the couple and ate her whole ( the woman had the bow in her inventory when she was devoured so in case you were wondering, yes you have to kill the monster to get the bow) the man survived and wrote in the journal. The journal says that a couple went hunting for a monster called Uderfrtyke Matron. Opent the sack an read the journal there should be some familiar names if you've played Morrowinds blood moon expansion package. When you get to dive rock there should be a tent and a sack near the edge of the cliff. It's difficult to get to because it is high up in the mountains, but I found a path by the camp of the master acrobatics trainer.

To get the frostwyrm bow, go to dive rock in the valus mountains, north east of cheydinhal.

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