I sort of feel like you're trying to do a little too much with this, and funny enough, I actually think that the overall concept still feels too weak as a standalone champion. More like someone who'd be there, be annoying, but essentially not really do anything until he hit 6 and pressed R.
taking only 10% damage, no matter the stipulation, is just outright annoying. 30% damage reduction permanently, with the stipulation that you have to micro your back to the enemy, is plenty. Braum's E (after blocking the first projectile) only negates 30%, and it ISNT permanent.
The Q just seems overly annoying, but also pretty much useless if you cant skillshot, or if they simply decide they want to kill you. It doesn't really need a reset mechanic, just needs to be your main form of poke, and leave it at that.
The W sounds sort of like Gul'Dan's Rain of Destruction in HotS: it could be extremely tilting if there's a time the tendrils just chain together on someone, but if the enemies just navigate around the tendrils, it will basically do nothing. Even with the passive of grounding someone who CCs you, there's simply no way they don't just stun you, break the channel, and put this W on what I'm going to hope is a decent cooldown. All in all, this ability really seems like it'd be pretty weak, as long as these tendrils weren't heat-seeking, perma-rooting machines...in which case, it's still be decently weak, cause they'd just stun you and put you on CD.
E is arguably the most imbalanced I think, although being able to turn off someone's BUFFS and being able to essentially take all their bonus stats away for a moment is definitely a unique concept. I like the way you phrased it as "bonus" stats, meaning that the normal stats of the items aren't affected. The shield during most teamfights will probably be irrelevant, even as full tank, as if they're able to catch you and damage you to the point that it procs, they can either kill you through it, or just wait a second and dash on you when its gone. The one major gameplay issue with this E is that certain items would literally be completely useless to build against this champion...Rod of Ages and Muramana being the two biggest ones affected: losing all that damage off either of those items in a fight would be extremely crippling, and in the case of RoA, you'd effectively being taking a huge chunk of an enemy's health AND mana in addition to their damage. So as much as I love the concept of it affecting items, I'd have to say you're not going to be able to go that route and have this champion concept see any love. If you were to forgo the item part, and just leave it as cleansing buffs and shields (jungle buffs not included) you would still have an ability that could potentially delete a fair amount of strength from the enemy affected.
The ult is definitely my biggest issue with this kit though. In its current state, it has two HUGE issues: assassin funneling and a complete lack of interaction. If you pop this on yi or yasuo, for example, they just autowin: their stats would be far too high for far too long, and they'd essentially 1v5 penta while you get 5 assists for just pressing R and walking away for a drink while they did all the work and waiting for your respawn. And in addition to that, in its current iteration, no matter what you do, you look like you're feeding. I would simply recommend a new ult altogether.
Personally, my suggestions for changes would be as follows:
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change the passive to negate 30% of damage taken from behind. This is comparable to the damage reduction from Braum's E, but permanent as long as you're able to properly micro your back to the enemy during their attacks, so this sounds more balanced. You definitely don't need any more damage reduction than that, so let's also remove the dmg redux from your W.
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Looking at your Q and W in their current states, I'd like to suggest a bit of combo potential, to make you feel more involved and rewarded for pulling off a play correctly. Leaving the passives alone for the moment, let's look at their actives. Let's keep the Q as a skillshot that marks and silences the first enemy hit, but take away this weird reset mechanic you have for the moment. And lets go ahead and say that proccing the mark with an auto or ability causes extra damage in a small area around the target and slows everything it exploded on for a second. I'd propose a complete change of the W active: make a small AoE zone in a target area that roots enemies hit...basically make it Swain's old Nevermove. When you're looking at a kit like that one, basically you'd want to land your AoE root, pop your E to cleanse their buffs and shields from them, then silence them and proc your mark. Combos like that feel rewarding and powerful when you pull them off, and that's a lot of what makes these types of champions fun to play.
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You have a LOT of passives. I sort of feel like you need to take some of them out. Specifically I'd suggest your W and E passives just be left out: the W passive really isn't all that useful, because if they cc you, they're still going to kill you: grounding them will not stop them from just walking up and beating the hell out of you. and the E as previously mentioned as basically worthless if they're already deciding to kill you. As for the Q movespeed bonus, we can put that onto your W: when you land W on a champion, gain a short-lived movespeed bonus. This means you can root them and quickly close in for your combo, or root them and give yourself more distance, depending on the situation.
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Since a lot of my suggestions are forming your concept into a utility/combo mage of sorts (the kind that can go support, but also can stand on their own) I would suggest a form of utility on your ult as well. How about this, sine you're seemingly about spectres here: "Extend five spectral tendrils in a long cone in front of you. If a tendril connects with an enemy champion, they take damage and are pulled back to you." This would mean that you can engage a fight from a fairly large distance away, and you can catch stragglers, but they can also try to counterplay by simply walking between the tendrils. It's basically like shooting 5 Blitz hooks at the same time, and as we all know, if blitz pulls you into his team, you're generally dead.
These are all just suggestions, and I feel like I've just absolutely reamed you here, but you do have some interesting concepts...I just don't think Riot is in a position to be ready for some of them.
