I don't think the KoS made any plans, per se. To use an overthought comparison, the KoS is akin to a Lovecraftian god, potent but remote. The shadow priests are cultists who revere the KoS in the same way Lovecraft's cultists revered Cthulu. They follow him because they know he'll come anyway, and when he does they might be spared destruction. But like Cthulu, the KoS doesn't really concern himself with his cult's aims. The cult worships him, but their interests aren't his.
Black Garius sought out the KoS out of a lust for power. He knew that the KoS was powerful and overestimated his ability. He thought he could steal the KoS' power and discard him, and instead he became enslaved.
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Exactly! ^_^