Many lifetimes ago Muir and Thayan had an alliance with another good god, Altius. He was a triumvirate god, with each part representing a different aspect of good; Lawful, Chaotic, and Neutral. Muir, Thayan, and Altius were at war with an evil god, who they managed to banish, but not before he and his followers split Altius and imprisoned him within three mortal bodies.
These men tried to hide themselves while they figured out how to undo this terrible deed. The Lawful aspect, however, couldn’t resist the opportunity to do some good in his state. None of them realized that they would live forever unless slain and he didn’t want to waste his life. This was Karith. The other two reached out to the Knights of the Inner Circle who know as much about prophecy and divination as my people. They searched for a ritual to put them back together.
Before they found it, the unthinkable happened Karith was slain. The forces of good shattered his soul so it couldn’t fall into the wrong hands. The prophecies indicated it would be some time before they could be safely reunited and they knew it could be used to bring back their nemesis. The Githzerai Book of Infinite Knowledge, Kerrasone, and a blade that can only be wielded by a paladin of Muir are required to bring Karith back.