Poseidon n Amphitrite

Series of expanding empires that swallowed each other, each with a ruling Kizier supported by various other mages and defended by bodyguards, the Tharii, originally an elite military unit.

At some point a particularly paranoid Kizier worked a spell to bind the other Kiziers and the Tharii to him, which basically rendered the fighters into mind-controlled slaves. This was not a great move and did not make a lot of friends, but nobody was really in a position to complain for a long time, as they were almost as stuck as the Kizier was.

Eventually there was a chosen one type - a young mage who came from a minor court family was in love with someone who was chosen to be a Tharii and quested to free their lover and bring down the evil empire. The backlash of "breaking" the spell created the modern system, as the mage was unable to destroy the bond but could weaken it.

Then all of the kizier and tharii souls were still bound, but instead of being bound to the emperor and unable to escape, they are bound to each other. Unable to escape that bond, they reincarnate and come together as pairs and reincarnate again. Now the bond went both ways, and often the kizier was more dependent on the tharii than the other way around.