What kind of commander disappeared, and what turn was it? There are a few magic things that can make units die (e.g. Burden of Time) or defect (e.g. a Succubus) without a specific message.
I'm not familiar enough with Ry'leh, but I know some of their summoning activities can drive units insane, especially if they aren't the special type that is supposed to do the summoning. That I think happens during the summoning though, rather than during a battle. There are some spells and weapons that can drive people insane though, I think.
This in addition to what PvK said:
I've posted on disappearing commanders before, which has happened IF they are stealthy and IF they sneak into independent territory and IF they autosummon units (e.g.: ghost king, Pangaian Harpy Queen, etc.). Could that have been it?
The commander was the Centaur Heirophant(sp) but he was not my prophet.
Basically the castle he was in was attacked twice in one turn (somewhere around turn 30-40) from the Messages and the battles images he survived but was gone at the start of the next turn.
I noticed because he had heroic strength which I'd never seen before and I had equiped him with the lightning bow with precision items. He was my best fighter at the time.