The following is a direct quotation from an Oblivion related Wikipedia.org branch. I present this quotation to you because it is the basis for my knowledge, and I doubt I could explain its subject material any better myself:
"Umbra is a level 50 NPC who provides one of the easiest ways to get Ebony armor in the early game. She is in an elven ruin you'll find brushing against the path if you follow the road from the Imperial City to Bravil; the ruin is named Vindasel. It is exactly at the point where the two roads meet and continue as one towards Bravil. When you enter the ruin, head to the very back and you will find her. She is not inherently hostile, and is not a "do not kill" person of quest importance.
If you kill her, you may loot her full suit of Umbra's Ebony Armor. You may also loot an incredibly valuable weightless longsword with 120 second Soul Trap, also named Umbra. You may keep the sword or turn it in as fulfillment of the quest given by Clavicus Vile, and be rewarded with the Masque of Clavicus Vile which boosts the wearer's personality by 20 points (constant effect).
If you visit Pell's Gate, as instructed in the quest, you learn that she once lived there as an apprentice to Irroke the Wide. She came across the sword, Umbra, somehow, which changed her, making her more aggressive and violent. She then took the name of the sword and left with a group of mercenaries to sate her blood lust. This evidently didn't work out, and she took up residence in the Vindasel ruin.
Killing Umbra counts as a murder of the innocent if you attack her before getting the quest from Clavicus Vile. If you aren't already a member of the Dark Brotherhood, this will get you an invitation."
I hope this article helps you. If you have any further Oblivion related questions, feel free to visit the always Oblivion related Wikipedia.org site attached.