The card reads Skaven caster or priest . This isn't perfectly clear, as it could a reference a Skaven disk (whether caster or priest) or a skaven caster and any priest, but I think we were supposed to apply the Skaven limiting condition to both keywords. Think of it this way: if I say middle-aged men and women I don't mean middle-aged men and all women everywhere. I probably mean middle-aged men and middle-aged women. If I want to be explicit, I should use the latter construction, but colloquially most English speakers use the former.
I think the same concept applies to this card. To me the card reads as saying that the originating disk must be a Skaven caster or a Skaven priest, so it wouldn't be able to trigger off of a non-Skaven priest unit.
I would also say that judging by the artwork and the way that they have released other faction-specific cards, this was meant to be limited to Skaven alone, regardless of the slight ambiguity. It was also printed in an all-destruction box, increasing the likelihood that it was meant to be destruction only.
That's my take on it, anwyay.
Edited by Jedhead