Like I said, C’thun could easily be subbed for any of the legendaries in this deck. I originally had him in it, but I personally dislike C’thun so I choose not to run him, but if you are missing any of the legendaries or just feel more comfortable with him in the deck, then by all means throw him in. He is in no way necessary to win as I have gotten plenty of wins without him, but like you said, he has a huge impact and can definitely win you plenty of games, I just personally get much more satisfaction from winning without using C’thun.