The Opener of Ways
A malevolent, half-mad, sentient planet from the far realms
If the Blood Prince is Hastur, then Allabar is Carcosa.
Imagine that the true evil is not the King in Yellow after all, but Lost Carcosa itself.
Strike that, Carcosa is the city, the City by the Lake, by Lake Hali.
The dark planet seems to be nameless, but orbits the star Aldebaran (Alpha Tauri).
See also: Barbelith, VALIS