Lore references for The Patient One

[Smegskull]

Other than Lords of madness does anyone know anywhere else this deity is mentioned?

[Tzardok]

Other than Lords of madness does anyone know anywhere else this deity is mentioned?

The deity was invented for the Book of Vile Darkness and was lucky to ever be mentioned a second time. Some of the other gods in there didn't even managed that.

[ShurikVch]

For what it's worth, "The Patient One" is also (according to the "Elder Evils: Shothragot (http://web.archive.org/web/20090619191816/http://www.wizards.com/default.asp?x=dnd/drfe/20080318a)" article) one of titles of Tharizdun...

[Smegskull]

In LoM he is made out to be quite distinct from Tharizdun. Being more a corruption and deceit focus than pure entropy (Reminds me a bit of the tenth lord of hell)

[Tzardok]

Tharizdun has, even if you ignore all those cults that revere him under some kind of disguise, so many titles that stepping on other evil gods' toes becomes unavoidable. For example, Tharizdun and Cyric share the epithet "the Black Sun".
It's possible that the same thing is happening with the Patient One. It's also possible that the Patient One is just one of Tharizdun's masks, like the Elder Elemental Eye or the Dark God.

[DragonIceAdept]

Lords of Madness is more ambiguous but still leans into a potential connection (bottom of page 177 talks about a possible relationship between Ghaunadaur, Tharizdun, and the Patient One).