After killing the hostile version of Cohlien Frostweaver, he drops [Cohlien's Cap] which starts this quest. You must locate Cohlien's lockbox and use the cap in front of it. The lockbox will highlight as you mouse over it, but is unusable itself. After using the cap in front of the lockbox, return to Custodian Dieworth for your reward. After you receive the Cap from Cohlien and kill him, he will spawn as a neutral version of himself, and will no longer aggro you unless you attack him.