Hum, are you sure you mean "Phoenix Pyre" (the spell allowing the mage to resurrect) and not "Holy Pyre" ?
The latter is a Fire spell but is specifically designed against Demons, "Holy" damage I think, so Imps haven't a chance against it.
It was phoenix pyre. A fire1 marignon mage used a gem to cast it, a bit later he was killed by a pile of imps and went boom thereby killing most of his crossbow friends and my imps. He resurrected and was killed again by the remaining imps and my other troops.