Cracks Fingers
Hey hey, I'm just going to put in my 2 cents plus change here.
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Overall, I think I like the idea. It reminds me of Geb from Smite (and all the other hero/champion/creature/boss/fighter/adventurers that have skill sets similar to Geb). I believe it's a good concept, and I like that it seems like a higher damage yet less CC intensive Kennen. However, with everything I like...there are things I don't like. So, here we go.
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The passive is overpowered. Keep the slow and Nurf (heh) the damage, or keep like...83.3333% of the damage and remove the slow...or make it incredibly mild. Honestly, I think you could keep both if you made it like 5% to 10% reduced movement speed per 2 stacks for 6 stacks max. Then at 6 stacks deal around 60/120/180 true damage over 4 seconds (Yes, it is a mini-ignite).
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The Q is nice, but I'm thinking you could make it into a land based damage over time like Rumble's ultimate. Umm...(70/105/140/175/210 + [0.4 AP]) in magic damage (+ [0.2 AP]) per second for 3 seconds. Add a stack of incineration every 1.5 seconds and slow them by 10% for every second they stay on the inflamed area.
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W is straight up ridiculous. Let's get real. Mundo, Volibear, Sejuani, Darius, Garen, Amumu, any other tank I forgot to mention, never stood a chance against that W. Bring it down to 6/10/14% + [ 0.01 to 0.015 AP] maximum health in magic damage and maybe reduce the (long) cooldown (I would hope) by about 2 to 3 seconds, and you might have something.
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I like E....E is cool...it's like a mini-Vi ult...which is cool.
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The ult...gives me too much of a Kayle feel. If you were alright with something like "For 1/2/3 seconds Cinder becomes a spirit of fire, taking 25/32.5/40% less damage and dealing the damage mitigated (or half of it) to all nearby enemies [+ 100/120/140] magic damage per second." I think that would be sort of cool. I think you could even tack on a flat 10 - 12% slow if you wanted and no one would complain much.
I can see where you were going with something of a E-R-Q-W combo, and I think what I've suggested still preserves that. But, I feel as though this way no one will end up with an Annie or Syndra that starts off with Rylai's. The slows are still there (I think)...incineration is still there, and I feel as though you'd have a smoother time playing Cinder (as well as counter-playing) with these minor changes or similar ones implemented.
Honestly, I like the idea of the kit. It does call from guys like Syndra, Brand, Annie, and a little Kennen, but it's the fusion of them that really makes it seem interesting. If not for a few overpowered things, I'd say Cinder would fit right in.
These were only suggestions and comments from a guy who knows not the ways of professional leaguing.