The dwarven goddess, sort of. She is very mysterious at present, and the dwarves variously believe that she has died, abandoned them, or entered some kind of seclusion. Supposedly they do not worship her.
Tertia is mentioned by dwarven characters throughout the game. In particular, a dwarf at a shrine in upper Eustrin mentions that the party is free to worship whomever they wish at the shrine, and that "Tertia did not preach jealousy." The same dwarf says Tertia was goddess of "all the Earth, not just the rock" and implies that she should not be considered just a goddess of dwarves.