Who's Onision?

I genuinely have no idea who he is. I keep seeing memes and posts about him with people saying how him coming back to YouTube would be their last straw and how he's left his mark on YouTube forever but none of them expalin anything. When I put his name into YouTube I just get hour long video essays about him and they just confuse me more. From what I gathered he's not a good person but I've no idea what he's done for sure as when I look it up up get a bunch of things he's apparently done and I'm not sure what is true or not.