This place unlocks during an optional quest for Karas, it'll guide you there eventually. The Master there is south of the entrance.
Also sorry for all the questions, should be the last one, but how does one go about progressing the quest for Karas and Slith? Benno's specifies a location, Matthis's is hinted at in dialogue, but those two seem to have no real leads. I've unlocked the ability to go to Falgar's castle and want to do all the side stuff before that.