Hello!
As a quick review, here are some suggestions and some insights i see about your concept
Overview
As a quick overview, you can add a bit of formatting to help with both presentation and readability. Not to say to have to put hours into it, but it could help. Here is a good post on all the formatting tricks and tips
Lore
Well the ole revenge plot driver. Nothing wrong with that, especially as there is additional pain to the champ by actually taking revenge with the missing memory/losing humanity. This i like. And something i think could play into the game visuals as he changes his looks based on the amount of kills he has. And a good way to have fun (if you decide to) with his voice lines changing throughout the length of the match. And generally going off of his title being a warlock seems like what is making him is trying to take over and control his body. Which i think is something that could be designed into the abilities. However, the "blood magic" route is a very close proximity to that, while being a understood concept and a existing mechanic in game.
Abilities
Without exact numbers it does make it a bit difficult to judge things. First and foremost you must state if your champ is melee or ranged, and the range of the auto attack. It also helps judge the value of the abilities. But guessing that each non ultimate spell should cost 15 to 20 resource and regenerate between energy and mana per second as a good guess, say 5 per second regarding the 100 total pool. I think the issue with this and the fact that you pay half in health is that the cost is very low. You'll probably want greater cost, at a pool of say 400. This greater cost per ability will then make casting at low resource a actual consideration, and with the additional damage bonus, a risk one can play towards and around. I would say that the additional damage from missing health is a absolute neccesity for the passive, as well as the kit.
As for points of clarity, you need to say with is the threshold of when you cant cast before you kill yourself - like if it will kill you, does it just reduce you to 1 health, or you just cant cast at all below X% health, and so forth.
This is a nice ability that doesn't base itself on the third hit mechanic. And it also allows for risky play since it heals you based on dealing damage afterwards. It also seems like a recovery jungler like xin and ww to help with his clear. And that is a difficult style to design for (in my opinion) and props for going that that route. AS helps with DPS and the heal for sustain. Overall i like it. It does however take away alot of his ability to duel. Particularly anyone who is not a full tank. 3 seconds is a long time in the late game so staying alive in a team fight to get both the damage and the heal will be tricky. This is doubly hard with any bit of cc focused on you. Its a great design for the jungle and will be (should be at least) punished if not used perfect versus a player. And a well combined skill with the nature of his passive.
Great ability here. If you want to somewhat provide use to hitting a champion, i would say any enemy champion damage will be Sighted untill they move a medium distance away, say 600 units or untill the skill is or 6 seconds
You are really putting risk on the player i see. I think this goes great with the kit so far. I do think this skill should be a barrage skill, allowing for the amount of risk to be determined by the player. Have a small dot, that is additive with every cast. Of course that will move it from less of a single nuking burst skill and more of a heavy bombardment. Even if the barrage count is two, i think that will be a improvement as the bonus AP could be used with more spells being that the Q is so aa focused.
Here is, along with the W is where you begin to stand a chance with a duel. Granted a stun must happen or your already low enough to deal enough surprise damage and kill them. But this is probably best casted from out of vision to help with connecting since its slow moving. Which really helps with since your a jungler. I do think the projectile speed should double at maximum range on the return to help with the chance to stun. Or maybe change it two a two phase ability allowing it explode and stun in a small area. This also allows for a instant shield instead of a probable tiny tricking shield. If you do decide to keep it as is, be sure to refresh the shields total duration every time it hits a new champion.
Now this is interesting. But i think you need to clarify if your not having any health regen while this is active. I have the feeling that might be the case, but its not worded as such. If its allows health regen per second, then the numbers need to make it so health regen items dont completely negate the drawback. Not doing so makes it all benefit with no drawback - unless you have to only itemize health regen lacking any damage items.
As a suggestion, i would say increase the spell range and cdr, as to further empower the warlocks magical prowess, along with the increased corruption regen. Maybe you can also have every x spell casted while toggeled on have 20% / 40% / 60% spell vamp. Another idea place Greivous Wounds On him. Maybe take it off on a kill. You can also encourage the player to keep the ability toggled on by reducing the constant damage to a lower amount. Overall this ult really rounds out the whole kit and reinforces the blood magic archetype going on.
Final Words
Numbers will help finish this concept. I like it overall and feel that it would fit into league just fine... after alot of play testing and balancing.
