The Tral Katra were an aspect of Vulcan society.

They were the secretive keepers of the spirits of the past and resided at the Fal-lan-tral on the planet Vulcan near Mount Seleya. The secrets of Tral Katra was never revealed to offworlders with many rumors existing about them; some of them held that the Tral Katra were enormous crystals which held the wisdom of Vulcan's departed whilst others claimed that they were Vulcan priestesses who sat in yearlong meditation's as they held the Katra's of thousand's of dead Vulcan's.

In 2373, Spock along with James T. Kirk journeyed to the Tral Katra in order to make peace with the disturbed katra of Sarek. (TOS novel: Avenger)

The novel does not actually explain what the Tral Katra is and does not go further into Spock and Kirk's journey to the Tral Katra leaving it up to the imagination of the reader on what they actually encountered in the cave.