Embrace Rock means in slang: A large boulder fell during Episode 6 Series 4 of BBC’s ‘Merlin’. The rock “allowed Merlin to embrace Arthur”.

The Embrace Rock was named after Arthur, who climbed down on his horse to embrace Merlin. After all those years, Arthur finally hugged Merlin. This was an important moment in Merthur’s history.
(in Slang Dictionary, added by Tania Kane)