What happened to Mu Enkai?
Well, if you will read the prologue to Ghostwater I think that's very clear that Mu Enkai fought Lindon and lost. And then he used that loss to motivate himself and fuel his rise. It was a meteoric rise likes to which has never been seen on cradle before, it makes Lindon look like nothing and he became a Monarch in the matter of days. He immediately ascended, joined the Abidan and became the 9th Judge. I think that's obvious...I thought I had implied all that in Ghostwater prologue so it's not even a spoiler.