Awesome Champ concept, went imagining his playstyle as I read through the entire post, and I seriously like it.
The only thing I dislike is the Scythe throw, I feel it is a bit limited since it needs a target.
I feel it has way more potential than that.
It should be a skill shot since then you can use it as an escape or chasing tool, plus it would be better as a skill shot since the ratio + damage on it at level 1 is quite strong especially for the utility it already brings.
Also I remember one of the rioters stating that mana burn might be a toxic thing to gameplay, refer to this: http://na.leagueoflegends.com/board/...25679#24925679
Perhaps instead of burning mana, it should silence champions in an area for 1.5 seconds or something, we haven't had something different like that in ages, would be a decent counter to champions like Amumu, galio, fiddles (when casting), kat, etc.
( Since it is an ulti, make it still usable when stunned but not while silenced )
Also, I feel that % based lifeseal is very powerful, if not too powerful, perhaps it should have a flat value + AD scaling or something, unless the other solution is lower it by 2% but make it scale very mildly with AD.
Perhaps you should add a small intro/entry about his general playstyle.
I'm just throwin you ideas that may improve the concept overall.
I really like it and feel it has potential which is why I'm giving you this feedback.
BTW, off topic question, where did you find that image of the reaper dude?