What happens if you silence C'Thun?
You can silence an in-play C'thun to revert it to a 6/6.
However, if a silenced C'thun later gets sent back to its owner's hand or deck, it will regain its buffed stats.
The aura still applies to cards like Klaxxi Amberweaver, which check for C'thun having 10+ Attack, as reported here.