They are the "mage hunters" mages who, desperate not to be killed by Malygos, betrayed the Kirin Tor and sided with them so they would have their lives spared. Some of them were enslaved apparently as well.

Malygos plays hardball.

In his own demented, probably still insane mine the best way to fight and eradicate the Arcane users of Azeroth is by unloading Arcane Spell users at them.

Crazy, right? Especially considering that his reasons are that the unchecked use of Arcane Magic attracts the Burning Legion like moths to a bug zapper. Only instead, the Moths are equipped with hand-grenades.

Anyway, when you quest for Amber Ledge in Borrean Tundra, you find that the Kirin Tor are desperately trying to find and free an Archmage who was captured by the Blueflight. If you haven't done the quests, I recommend the Tundra, as it (as well as the chains in Dragonblight) show how the Kirin Tor uncover Malygos' plot to siphon the world's magic and blast it off into space.

Anyway, between the Arcmage I mentioned before and Keristraza in the Nexus, it becomes painfully clear that Malygos is not above tormenting and warping people's minds to his will; it's not unlikely that Malygos did the same to many of the Blueflight who opposed the idea of warring with mortals and the other Dragonflights. All in all, Malygos isn't much better then Arthas when it comes to recruiting help.

plain and simple-quislings