[Taqwus]

As Jotunheim finishing up a single-player game w/ T'ien Ch'i being the only surviving AI (and one that had maybe 2/3-3/4 of the map, *shrug* I don't like blitzing, would rather play with magic), I did Wish for a Vastness, cast Gift of Reason on it, found out that it was Astral-3 (no equipment slots 'tho), and empowered it to Death-3. Mmm, Soul Vortex + Blood Vengeance + Nether Darts...

Vastness is likely based on Azathoth.
Otherness... might be Shoggoths. Read "At the Mountains of Madness". *shudder*
Formless spawn came from Tsathoggua, probably.
Elder things also can be found in "At the Mountains of Madness".
Others of their monsters, e.g. hybrids, come from "The Shadow over Innsmouth".
Some others I'm not sure about, but then I haven't read all that much Lovecraft / Derleth / etc.