What's his lore? That would be useful to know, especially given what we know (and don't know) about demons in Runeterra.
Also, the mana drain mechanic on his ult is kind of awkward due to not all champions using mana. Maybe change it to mana recovery, possibly scaling with targets hit.
You described him as a tanky-assassin, I believe the term for that would either be a diver or a skirmisher depending on whether he falls closer to the tanky end or the assassin end.
Passive: If I understand correctly, every sixth hit against champions heals and grants attack speed at the cost of mana.
Q: Is this a single target ability? If so maybe change it to an empowered basic attack for auto attack resets and to help proc his passive.
W: A basic dash with radial damage. Similar to Kayn's q in theory though it would be nice to get a general idea of the range and cooldown. This would be his tool for gapclosing, sticking to his target, and escaping and I guess it works though specifics would be useful to know.
E: Lets him trade health for mana and a partial cooldown reset. This is a fairly cool ability though it does feed into two problems he has that I'll bring up later. How does this work with his lore, why is he cutting himself with his regular swords?
R: Are these attacks single target abilities? Do they strike in a line of cone or something like that? Are they empowered autos? What's the lockout time between each cast if any? On a side note, why is the last of the three attacks the one that applies the damage over time effect?
There are two main issues I have with this:
First, is he focused around basic attacks or abilities? His passive suggests a focus on basic attacks but I can't tell if the rest of his kit supports that.
Second, he's supposed to be sort of durable but nothing in his kit helps him not die aside from a bit of innate sustain and his e actually makes him easier to kill.
Here's my suggestions:
Passive: Instead of a mana cost on the initial attack, each attack while the attack speed is in effect turns a percent of his current mana into damage similar to manamune.
Q: Is an empowered auto, maybe add some additional effect aside from damage, while passive is in effect it burns a much larger portion of mana.
E: something like a brief shield or damage reduction on use to help with not dying.
R: Is a targeted ability that applies on hits, gap closes a short distance, and has something like a 1.5 second lockout between uses. Poison on third hit is gone since the timing is supposed to be more tactical.
Basically, this focuses on auto attacks and doubled down on the idea of turning mana into auto attack damage.
