I'm new to D&D and am playing a warlock character with the Great Old One as a patron. I'm a bit disappointed at the lack of flavour that is available in the PHB regarding this and was wondering if there was anything else out there in 5e or any other edition (that is still readily available) that can give a bit more background on these Great Old ones?
The Great Old Ones is more of a group name for gods and monsters rather than the description of a series of similar beings. They are not the entities with the same name from the Cthulhu Mythos; instead they are a number of entities from a variety of sources. The ones mentioned are:
Ghaunadaur (Ghaunadar in the 5e PHB)
A member of the Drow pantheon, the god of outcasts, oozes, slimes, jellies and other such beings. Also receives lip service from Aboleth. Sources:
- AD&D2e: Drow of the Underdark, Demihuman Deities
- 3/3.5e: Faiths and Pantheons, Lords of Madness
Tharizdun, the Chained God
The god of insanity, entropy, dark knowledge and other such pleasantness. Is currently locked away in a demiplane serving as his personal prison. If he ever escaped, all gods of both good and evil would likely join forces to lock him away again. He is masquerading as the Elder Elemental Eye, a being of immense power worshipped by a fair number of evil elementals. Sources:
- AD&D1e: The Forgotten Temple of Tharizdun
- 3/3.5e: Return to the Temple of Elemental Evil, Complete Divine, Lords of Madness, Dragon #294 and #353
- 4e: The Plane Below. Note that Tharizdun's role here is a bit different (par course for 4e)
Dendar the Night Serpent
An ancient evil of Faerûn, she is a thing of nightmares and the harbinger of the end of the world. Dendar is a 300' long serpent with black to deep green scales, with a mouth capable of swallowing a dragon whole. She is currently located in the Fugue Plane, feasing on the nightmares of the dreaming in her sleep. Sources:
- 3/3.5e: Champions of Ruin
- 4e: Heroes of the Elemental Chaos (in a different role)
Zargon the Returner
A one-eyed horned monster worshipped by an evil cult responsible for the fall of the city of Cynidicea. Sources:
- D&D Basic: The Lost City
- 3/3.5e: Elder Evils. This iteration is held to a degree of contempt because it radically changes Zargon's origins and links him to Baator and Asmodeus, a link predating both of those as we know them now. He was also significantly depowered and is not so great a threat it can qualify as an "Elder Evil" or "Great Old One".
Ol' squidface himself. You know him, you love him. Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn.
- AD&D1e: Deities and Demigods, 1980 print. TSR (the then-owners of D&D) believed they could add the Cthulhu Mythos to the book because it was public domain, but Arkham House claimed copyright and that they had licensed the Mythos to Chaosium, who you might know as the studio behind Call of Cthulhu. The Mythos (as well as materials from Elric of Melniboné, whose license Chaosium also held) remained in the book as the result of a compromise between TSR and Chaosium, but they were cut from the 1981 printing because TSR did not want to make references like that to their competition. This makes the 1980 prints of Deities & Demigods very valuable to collectors.
In the campaign setting 'primeval thule' which is available for fifth edition they go into depth with a heavy cthulhu mythos. In the core cook and the pkayers books they have rules for clerics of old ones, different types of warlocks for different great old ones ( e.g hastur, yog sottoth, etc.), and spells with a strong mythos theme. Would recommend for a cthulhu flavour game