A couple of things

[PvK]

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.

PvK