There were once three Courts; Red, Yellow, and Black.
Rulers:
Lord of the Red Court, Knith-Hgor - Deceased
Lord of the Yellow Court, The Nameless King/Hastur - Deceased
Lord of the Black Court, The Mad God/Azathoth - Alive, but asleep
The Courts hated one another and fought to destroy the others.
Red and Yellow warred until their mutual destruction, while the Black Court merely watched from the sidelines and made sure Azathoth didn't wake.
Black is the only Court remaining.
If Azathoth wakes, reality is completely undone; it's torn apart and destroyed and all of creation will cease to exist.
Knith-Hgor's domain was War.
Members of his court: Unknown
Hastur's domain was Pestilence of known and unknown origins.
Members of his court: Unknown
Azathoth's domain is Death.
Known members of his court:
Yhoundeh, the Untamed Primal
Noth-Yidik, the Ravenous Devourer (neutralized)
Shub-Niggurath, the Black Goat of the Woods (neutralized)
Nyarathotep, the Crawling Chaos (neutralized)
Cxaxukluth, the Illimitable Desire
Yog-Sothoth, the Key and the Guardian of the Gate (Turncoat)
Ubbo-Sathla, the Unbegotten Source (neutralized)
Abhoth, the Source of Uncleanness
Daoloth, the Parter of Veils (neutralized)
Dagon, the Ruler of the Deep (neutralized)
Quachil-Uttaus, the Lord of Decay (neutralized)
Azathoth, the Demon Sultan