I'm just another one of those. You know, those who do things. Things that need to be done by those that do these things in order to do those things that don't do these things themselves. Has the word 'things' lost meaning to you already?
I can see it now... opponent plays C'Thun; your board gets wrecked but you live. Then you play this. And basically sacrifice their C'Thun to appease your C'Thun. Then you play your own monstrously huge C'Thun, and the servers crash.
So it loses 5/3 from Kraken, but it can effectively deal assassinate rather than eviscerate. So it gains 3 mana from its effect ando loses about 4 mana for stats. Still the buff effect probably justifies it. You just have to make sure you tempo outC'thun right on turn 10 to make use of this tempo or it'll be for nothing.
Potentially very strong in a slow meta and has incredible synergy with Shadowcaster. C'Thun rogue starting to look pretty interesting...assuming Rogues get some half-decent board control mechanics to survive until lategame!