You must have missed that explanation completely lol. The King was in a death state and Rood just happened to be one of the random corpses that had been 'loved' and 'tossed' and was willing to 'sacrifice' himself for a 'greater' good to the tower his elder brother worked for. The king wasn't dead, dead just had no body; which is odd if you think about it, but meh it's a work of fiction not much is meant to make sense.
But anyways the King used Rood's body as a vessel, but his memories were locked away and he would live on as Rood since his brother did jump in to get him. It was quite a handful of chapters ago when you were learning that history part. That king was giving the brother another chance with Rood while also gaining his own second chance with a new vessel that as the years went by grew to adapt to his soul and magic.
Understandable?
So does that mean there's 2 souls/people inside that body? Does that mean when the time comes one of them would have to fully vanish? Nooo my poor baby Rood and sexy King Blow ╥﹏╥
Like maybe when King Blow finally has all his memories or powers back or something? So either the king will sacrifice himself and so Rood can live fully or Rood will sacrifice himself so the king can live and bring harmony to his people? Oh the tragedy
(is this considered a spoiler? i don't really know but oh well, If you just started reading, #SPOILER#)
... why did King decide to take over Rood's corpse and come to the human world? (ö . ö ) Is it still to be explained or did I really miss something so essential?