Set was a "dark" god (chaos, violence, fire, storms, etc) who was worshiped in historical pharaonic Egypt. He was the dark god / evil god whom was worshiped by many of Conan the Cimmerian's sorcerer enemies as well as in the dark kingdom of Stygia, mentioned for the first time in 1932 in Robert E. Howard's "The Phoenix on the Sword". In 2018, Modiphius Entertainment, the publisher of the pencil-and-paper roleplaying game "Conan: Adventures in an Age Undreamed Of" rectonned Set into a Great Old One in the Nameless Cults sourcebook.
Set who is known by many names (Great Dragon, Yig, and Quetzalcoatl), is a slavering hungry deity. It's from the Outer Dark, a being whose physical form is rejected by the world, twisting reality around it. Relics which have been blessed or touched by him are no longer completely part of the physical world, which is the reason that they are endowed with incredible powers. Any priest who draws on those powers does so at great personal risk, as the forces that are unleashed corrupt the flesh and tear at the mind.
Apostate Stygians who speak blasphemies against Set often meet sticky ends. They mysteriously disappear, are found hideously murdered via the most inhuman methods, or they end up slowly turning into serpent men as their eyes turn yellow and their skin slips away. Those who tend carefully to serpents appear to earn its blessing. Human sacrifices are regularly dedicated to Set and so are the mysterious rites. It's believed that he is a Great Old One that is sustained by blood.