Okay, I see several problems with this champion concept. I'll offer my thoughts on what I don't like at each part of the champion. This is intended to be constructive criticism, so please don't take offense to anything I say.
Appearance: You don't provide any description of Gorgon besides the entities that hover over his shoulders, and even then you just say what color they are. The void creatures all have wildly different appearances, so is this guy animal-like in appearance like Rek'sai or Kog'maw? Is he human-like in appeance like Kassadin or Malzahar? Or is he some sort of floating creature like Vel'koz? Appearance is a big part of the character concept; you need to give us something to picture when we think about your champion.
Passive: This isn't really written clearly, so I'm not quite sure what you're intending here. I'm guessing it lets you choose an entity, and if you are using that entity, the cooldown of Q wil be reduced by 3? That seems pretty weak, compared to a lot of other passives in the game. Also, when in the game would you be able to choose? Does the choice need to be made at level 1? Could you wait until you were a higher level to choose? Can you change your choice later in the game, if you decide that you want to change up your playstyle? Personally I think going with a different passive is for the best, but if you really like it, keep it, just word it better.
**Q: **Okay, so this isn't just for your Q, but for all your basic abilities. You're only listing damage, cooldown, and mana costs for 4 levels, but each basic ability in League has 5 levels. You definitely need to fix that. Moving on to the Q itself, I actually like the idea of it. However, you're not too clear on the range of the entity, and how many times it will ricochet. Will it go until it hits a wall? Does it travel the length of the screen and then return? Can it bounce between two walls infinitely if you shoot it at the right time? Can you only fire one entity at once? Does it damage everyone in an AOE from where it hits? Or does it just go through champions, damaging everyone it hits? Can minions block it? The damage is very weak too; comparing it to someone like Vel'koz shows that it does the same amount of damage at rank 5 as Vel'koz's Q does at rank 1. I do like the idea of being able to choose whether to do more damage, or sacrifice some of that damage for a heal though. I think you should definitely keep this ability, just fix the levels, damage, and word things more clearly.
**W: **This is a pretty basic ability, and I think it's pretty good as is. Mana cost is a little high, and I don't think abilities like this usually have mana costs attached to them. Maybe make it have no mana cost, but have the cooldown scale down based on your rank in the ability.
E: I like how this increases the low damage of the Q, but it doesn't really seem that great. Like I kind of see what you're trying to do; you're trying to make your champion be mainly based around 1 ability, kind of like Azir. But Azir's soldiers are far easier to get out and far more useful than your entities, and his Q and E actually deal damage and have other effects. In contrast, your champion's damage comes 100% from his Q, and if you end up missing a Q, you can't deal any damage until it comes off cooldown. With other champions, you'd have other abilities to deal damage, but Gorgon is way too reliant on Q. The E is just too situational and weak to be effective, and I think you should consider changing it completely.
R: The ultimate is just a huge mess. I don't really understand why a mage would get health regen from one of their abilities (it's more of a tank thing), and you don't specify what gets more damage. Does he do AOE damage when his ult is on? Do his autoattacks do more damage when he's ulting? Can he still use his Q when he's ulting? How can he, if his entities have merged with him? I think it's a cool idea to have a short cooldown ult that drains mana per second, but the amount drained is pretty low (Anivia's ult costs 75 to activate plus 40 per second at rank 1). I definitely think you need to rework this.
Overall: Honestly, your champion is incredibly weak and would probably not be able to survive in the current mid lane meta, let alone any other lane. You claim he's a mage/assassin, but he really isn't very good at either. Literally every champion in the game who has Mage as their primary role has some form of CC, even if it's just a slow, but Gorgon lacks any CC. Why would someone pick him when they could pick someone like Annie, who can stun the entire enemy team and help win a teamfight? He's also a pretty bad assassin, because he has absolutely zero mobility. He can't go into a teamfight, get past the tank line, and assassinate a squishy ADC like Leblanc can. He definitely needs either some CC or mobility on his kit. But even with added mobility or CC, I think he's too mechanically complex and single-ability reliant to be a viable pick at all. You definitely need to work on this a bit more.
