Just to set the record straight—Kaan had never been possessed by Thessara. The Eleventh Ancestor wasn't in the business of body-snatching, nor did she have the ability to do so.
Instead, she paid a hefty price (with the amount of worship she got, she's a billionaire in the supernatural world) to drag herself down to earth and have a few words with her hand-picked descendants.
You know, the same way the Fallen One had been whispering naggings and bad jokes into Riona's ear.
Of course, not just anyone could make the cut. Take Klyn, for example—way too nice, way too righteous, and honestly, just too much of a pushover to be Thessara's little villain-in-the-making.
Kaan, though, had just the right mix of ambition, cunning, and moral flexibility. He was exactly the kind of guy Thessara could count on to carry out her world-ending revenge tour.