In the Titus Crow stories by Brian Lumley, Cthulhu's daughter. Fathered, along with three sons, 'upon a female sentience from remote, ultra-telluric Xoth, the dim green double sun that glitters like a daemonic eye in the blacknesses beyond Abbith...'.
Along with three sons, Ghatanothoa, Ythogtha and Zoth-Ommog. Cthulhu, according to Lumley, was using Cthylla as insurance, should he himself ever be killed. He intended to become reincarnated in his daughter's womb, fathered by Dagon and nursed by Hydra.
Cthylla is generally referred to as the 'Secret Seed' of Cthulhu. The following couplet is used in Cthylla rites to refer to the process of reincarnation, along with That is not dead which can eternal lie:
The dreamer dying faces death with scorn,
And in his seed will rise again reborn!
From 'The Transition of Titus Crow', by Brian Lumley, ch. 5: Cthulhu's Cosmic Miscegenation.