Hira said Ashura-ou, so I'm assuming she means King Ashura from Fai's Land, not the Ashura with the long ears!
There's nothing being said about what Ashura-ou in Celes World did that makes Fai seals him up at the bottom of the sea. Maybe it's just a promise they made together out of good intention like RG-Veda, or maybe it's Fai's emotional problem due to Ashura's abuse
Actually this has been discussed a ton in different threads.
My own idea is this:
Fai was the King's wizard and lover. Ashura had always been extremely controlling and
somewhat violent and such, but I think maybe he suddenly became worse, actually using
his power for evil, getting out of control. I believe he tricked Fai into doing many things that
he is now extremely guilt-ridden about, so doesn't care much for his own life at this point.
I think that Fai did love Ashura, hence the look on his face as he leaves Celes and Ashura behind.
I think too though, that Fai feared Ashura very much, and is afraid of what will happen when Ashura shows up. I don't think he's afraid of being killed, but perhaps something else.........perhaps controlled, or reminded or something along those lines.
The bodies of the men laying dead around Celes. I have a suspicion Fai may have had something to do with that. Perhaps trying to protect Ashura.
That could be one thing he's feeling guilt over. That he killed for Ashura, when in reality Ashura was becoming evil. Perhaps Ashura has a feather.
I also think Fai sealed away Ashura instead of killing him, perhaps not being able to kill him because of the feelings he still had for him, even after all he'd done, or sealed him out of self defence.
I'm not sure about that one though, because I think at that point Fai would have probably let Ashura kill him. And Fai's markings. I suspect they were put there by Ashura. But I don't know.