I can't really say who got who in the palm of their hands, but I believe it's Eyra. I love their dynamic so much
Yanu's obsession was sudden but it made sense. Eyra was cruel and merciless in their battle and was fully prepared to kill Yanu by any means. Yanu had never seen those sides of Eyra before - him lying about his position as the Lord, evacuating his territory and preparing for battle, using Yanu's sympathy against him, and even using the one weakness he was aware of to do some damage. To a powerful being like a dragon, Eyra's existence and nature are both enticing.
My guy folded with just a dragon eye and the pearl im guessing it's to bound him forever to only him or to make him lay dragon eggs lmao though it's sudden how yanu went from obsession 2 to 100 after that battle with eyra... and now after pursuing for a while he got him on his palm already