A new patch will be released soon to fix a bug in the C’Thun encounter, rising it to Patch 1.12.1 (Build 1).
Tigole: “We’re in the process of investigating C’Thun’s behavior in Patch 1.12. Unfortunately, we won’t be able to fully fix the encounter without a new client build. We’re hoping to get a Patch 1.12.1 together in the next few weeks that will fix the problem.
In the meantime, we’re working on a Hotfix that will temporarily give players more time to enter his room without suffering extreme consequences. We hope to have this hotfix up soon.
UPDATE: “A hotfix has been applied to all realms which should make entering C’Thun’s room more feasible.
In 1.12.1 a cleaner, better fix will be applied. But for the meantime, the encounter should be doable.”