those close to you, so basically, anyone you do a favor for. Could be followers, could just be random quest giver (I think). But I'm also pretty sure that any person you can marry will count after "earning their favor" (doing their quest)
EDIT: and apparently it's 10 kills, not sure on that, but you will get a message from Mephala(I think) when it hits max.
Pour me another, 'cause I can still see the floor.
This weapon comes with a 10 point life leech and can be upgraded by slaying your friends (i.e., someone you've completed a favor for, completed a task for, or won over during any type of quest). For every two such characters killed the weapon increases in power, adding two to the strength of the life leech effect up to the maximum of 30.
Contrary to it's classification, this weapon benefits only from one-handed damage item enchantments. Only one-handed perks will affect it's damage value. The mystery of this perk mismatch is only deepened, since a character with a properly filled Two-Handed perk tree and an untouched One-Handed can still perform decapitations with the ebony blade.
Using the Ebony Blade can benefit characters who want to continue leveling up through weapon advancement but have maxed out their one-handed weapon skill. The weapon allows for a character to train the two-handed weapon skill without sacrificing damage or having to change gear. However, for characters that use two-handed weapons normally, this weapon is a poor choice because of its skill/perk mismatch.
I just finished upgrading it (Mephala told me so), and all I did was hire Followers from Inns. Hire Mercs and other followers (such as people you Brawl with to gain respect) and then take them to an empty house and just sneak attack the heck out of them. I also killed my Housecarl in Markarth.
Took me probly a half hour to get the 10 kills needed. Oh, 2 of the kills were the Riverwood Trader people. I'm the Thieves Guild master so I bribed the guards to look away for 1000 gold