The Oh God of Hangovers. His believers, if they can be called as such, are the people who wake up after a night's drinking and moan "oh god...", as they suffer from a hangover. Bilious was created from the excess belief that spilled over when Teatime killed the Hogfather. He acts as though he has existed for a long time. It is possible that he was more of an omnipresent reservoir for hangovers before his manifestation, because something had to be getting the hangover.
Bilious continuously suffered from a severe hangover, with headache, nausea, vision problems, fainting, and more. He looked like a young man with the classical toga, but his toga was colorful with bits of things that have come up out of him that he has not eaten or drunk the night before. Although hangovers are common, he was relatively minor among the Discworld gods. He hated people who drank alcohol. He hated most of all the god of wine, Bibulous, who was probably not even aware of Bilious's existence. Bibulous could only feel the positive effects of drinking wine, while all the negative effects were transferred to Bilious. The wizards (correctly) reasoned that the reverse would be true: if Bilious drank a hangover cure, then he would only feel the positive effects while the negative effects would be transferred to Bibulous. They create a potion composed of many things that have some vague effect on hangovers, including Wow-Wow Sauce. The potions cures Bilious of his hangover, as well as getting him revenge against Bibulous.
Many of the minor gods created during the Hogfather's disappearance went away once the Hogfather returned. It is not clear if the same thing happened to Bilious. Violet Bottler's belief in him may keep him going, as may the fact that gods can never truly die, even if they have no believers (Small Gods). However, if Violet's interest in Bilious was as shallow as the Senior Wrangler's interest in the Cheerful Fairy, who did disappear, Bilious may've suffered the same fate. It is also known that the Eater of Socks has survived in the Unseen University so there is a chance that Bilious is still around.
Appears in Hogfather.