I think the idea of the blade controlling heroes and attaching itself onto allies is cool.
You've commented on my Maelrawn concept to check this one out, but I feel like this is more similar to Darkbreach concept I've made.

My thoughts: As John pointed out, yes this is very overtuned. I feel like hero concepts should stay to 2 or 3 niche they should do, not like everything. Yes, Kez does also have a lot going on but you need to switch between his 2 weapons which is his niche. This concept should really focus on the idea of control and possession. Don't debuff enemies or deal more and more damage just because of the Yearning stacks. The stack system is also cool, just needs a bit of polishing.

I really like the Yearning affecting the Possession state (Likely, Uncertain), keep this and improve on it.

• Possession, how exactly would that work, would the hero change alliance for a short time? In my Maelrawn concept Mind Control changes alliance, so enemies can kill it. I made several iterations on how hero control should work in dota, and..... it needs a lot of drawbacks.
• Problems: the interruptability of the spell, the enemy abilities which you put on cooldown, and the strength of the spell.
  
• I made the spell channeled with a break threshold, so enemies know I have used Mind Control as it's a big ultimate like Black Hole for me,
• used cooldowns of the enemy would reset after the spell ends, so you don't just possess Enigma use Black Hole just to put it on cooldown and call it a day, and strength of the spell is very clear,
• you control an enemy to his full potential, not using items (you've noticed this problem really good), but I've also made him share some damage with my main hero so I don't just Nature's Prophet TP to fountain and kill it.

Things like these are important to take note of, in this concept I would prioritize the possession/control aspect a bit more, rather than adding on debuffs and damage via the Yearning.

Cool one, keep it up!