User blog comment:Miskos3/Fairy Tail 528: Dragon of Magic, Review/@comment-29778526-20170325125701/@comment-30462872-20170326081217

Acnologia described himself as the 'Evil Dragon', and he's pretty much been the Grim Reaper of the Dragons the entire time. If his magic is just purely Energy, with it as the only attribute, he could theoretically rip out souls because at some early point it's stated that Magic is a spiritual energy, so it could be produced from someone's soul.

As it is, the dragons seem far more attuned with Magic than humans, given they can wholly transform into it, change memories essentially at will, steal memories like it's nothing, implant fake ones that are so realistic it's impossible to tell that they're synthetic, produce huge Roars even on the brink of death in Igneel's case. A little theory of mine is that dragons are essentially magical creatures, it's their lifeforce, which would be why the reaped needed a dense Ethernano atmosphere to gain back what strength they did in order to destroy FACE and attack Acnologia.

If they are dependent of magic for life, Acnologia could of ripped the magic from their bodies in such a way that he stole their souls and damaged their bodies beyond all repair. The damage to their bodies would cause them to slowly die as their wounds, despite being physically healed, would catch up to them.

Also, why didn't Igneel have the rest of the dragons come out sooner and help him face Acnologia, it would have made so much more sense. Unless he was so confident is his power as the Fire Dragon King, or it was a personal vendetta he had against Acnologia.