It’s a part of the main storyline for the expansion, and is picked up during the Symbols of Office questline, which has you taking down a lookalike doppelganger pretending to be you. Shadowrend, from TES IV: Oblivion– Shadowrend is actually not found in the base game for Oblivion, and instead is a part of one of the game’s DLCs, Shivering Isles. Beating her nets you some sweet armor, and this very powerful one-handed sword.
However, if you know where to look you can find Umbra early on and defeat her, and since she’s level locked at 50 she drops the weapon no matter what level you are – so at level one you can traipse over and take her out – just make sure you bring like, a bajillion arrows to try and whittle her down from afar, because she’s a tough cookie. It’s dropped from a character named Umbra (shocking, I know) and is part of a long questline that happens late in Oblivion. Umbra, from TES IV: Oblivion – Umbra is one of the most powerful weapons in the 4th installment in The Elder Scrolls franchise and it’s one that you can actually get super early on in the game.
