Idh-yaa, also known as Quum-yaa, Cthulhu's Mate, The Mighty Mother or the Xothic Matriarch, is a Great Old One who features in the H.P. Lovecraft-inspired Cthulhu Mythos.
According to the Ponape Scripture, Idh-yaa is a colossal, pale, worm-like monstrosity which dwells beneath the surface of a planet in the green binary Xoth star system. She is not only Great Cthulhu's mate but also his half-sister, birthing four offspring to him, the "sons" being Ghatanothoa, Ythogtha and Zoth-Ommog, and a single "daughter," Cthylla. When Cthulhu and his spawn travelled to Yuggoth on their way to Earth, Idh-yaa apparently stayed behind on her homeworld.
- Out of the Ages (1975), by Lin Carter.