Caerlanrigh

"The Caerlanrigh is the very centre of their powers."

- Gaius to Merlin about the Disir

Caerlanrigh is a cave located in the Grove of Brineved. It contains the spring that feeds the ancient pool used by the Disir to divine the will of the Triple Goddess, and is therefore considered sacred by followers of the Old Religion.

History
After Arthur received a runemark from the sorcerer Osgar, he, Merlin, and the Knights of the Round Table journeyed to Caerlanrigh to challenge the Disir's judgment. The meeting went poorly as the Disir condemned Arthur for his disrespectful behaviour. A fight broke out when Gwaine came to his defense and Mordred was mortally wounded, forcing the knights to retreat.

Arthur and Merlin later returned to Caerlanrigh to ask the Disir to spare Mordred. They agreed to do so, but only if Arthur would embrace the Old Religion. When Arthur refused, the Disir decreed that he'd sealed his fate and that of his kingdom and sent him away (The Disir).

Trivia

 * According to Gaius, the Old Religion is at its strongest at Caerlanrigh, as it is the very centre of its powers. The Disir also stated that the very heart of the Old Religion was at Caerlanrigh (The Disir).