The ebony blade is a katana like two handed weapon in Skyrim. Its apparently associated with one of the daedric quests, but which one, and how do I start it?
Also, once I obtain it, how do I power it up?
The Ebony Blade is the reward for the Daedric quest The Whispering Door. You can begin the quest once you reach level 20 and:
You defeated the dragon terrorizing Whiterun early in the main questline.
To start the quest, ask the innkeeper in Whiterun about any rumors he might’ve heard.
The Ebony Blade is a most sinister weapon. Its description says:
Strengthened by the blood of deceit.
To do that:
You must kill the people who you’ve befriended/done favors for in the ultimate act of betrayal. Every 2 kills increases its life leach .