Umbra Sword (Oblivion)

The Umbra sword is arguably one of the most powerful weapons in Cyrodiil. You can obtain it only by killing Umbra herself in the ruin Vindasel. This can be done at any point in the game, with or without starting the quest. It can be obtained as soon as you exit the sewers after being given the Amulet of Kings.

Until you complete the Shrine of Clavicus Vile quest, the sword has zero weight. The shrine quest requires you to obtain the sword and return it to Clavicus Vile, in which you will receive the Masque of Clavicus Vile as a reward, which boosts personality by 20 points. Or you can keep the sword for yourself and end the quest with Umbra.

Tips

 * This sword can actually be obtained early on, without the daedric quest, by killing Umbra.
 * Umbra becomes especially powerful when used with the daedric artifact, Azura's Star, as an easy way to recharge it.
 * With 100+ Blade skill, 100+ Strength, and 125% condition, Umbra can deal 33 damage per hit, the highest base damage of any one-handed sword (excluding weapons with damage enchantments).
 * It is flagged as a Quest Item, so you cannot remove it from your inventory.
 * It scales in power with your character.

Bugs
Umbra can be kept in your inventory with zero weight but it cannot be removed, to do this, simply tell Umbra during the daedric quest that you will leave peacfully, after the quest update has popped up telling you to return to the shrine, kill Umbra but do not take anything. Head back to the shrine and tell the god that you will not give him the sword. When you go back to where you killed Umbra, you can take all of her armor and Umbra's sword, the sword now weighs nothing and cannot be dropped.

Trivia

 * Umbra is the scientific name for the darkest part of a shadow.
 * Umbra is also latin for shadow.