The Cailleach

The Cailleach (pronouced as Kay-lyx) is the gatekeeper to the spirit world who appears after Morgana has torn tthe mystical veil between her world and the next and releasesd both the gatekeeper and her people The Dorocha. She is prortrayed by Gemma Jones.

The Darkest Hour
In The Darkest Hour Morgana sacrifices Morgause at the Isle of the Blessed in order to receive her power and in doing to tears the veil between her world an the spirit world, releasing the Cailleach. The Cailleach then warns Morgana of Emrys, who she says is Morgana's destiny and doom.

As Merlin is serving at the royal banquet for Samhain Eve she appears to him calling him Emrys, and he faints from the shock of it. When he comes about later, he realizes that he was the only one to have seen her.