User blog comment:B214/Volume 21 info & spoilers/@comment-82.199.217.179-20160328155617/@comment-73.160.58.215-20160328181137

Don't know. And there's no guarantee that its revealed in the novel (which I haven't read). I mean how they achieved that level of power wasn't necessarily as important as that they had. And it's not that surprising as they were only second to Crom Cruach amongst the Evil Dragons.