Elder Scrolls 4: Oblivion Message Board  

To get Umbra do the Clavicus Vile Daedra side quest. It's not actually the item you get for doing the quest from him, but you can keep it. At the end of the quest, after defeating Umbra, you can just leave the quest unfinished and the sword will not weigh anything. But if you want to complete all the Daedra side quests then you can return to his shrine and refuse to give it to him, then it will weigh around 45.

shadow rend is definately the best weapon in the game, and is almost always over looked. i know pretty much everything about oblivion, to get shadowrend, do the main quest for shivering isles and when your building your staff, you have to go to a tree and fight a shadow version of yourself. be sure to search its body because its carrying shadowrend.