User blog comment:LordLuminous/Fairy Tail 491: Mother and Child Review/@comment-5303512-20160629200401/@comment-4829046-20160701133006

I'm betting that August was referring to Serena's dragon lacrima, which is basically his entire fighting potential. Under August's hands, they will be far more useful.

It would just be bad writing for Acnologia to make a blunder and not sense that a DS was revived. Gajeel is a different story because he was in the middle of a warp when Acnologia last checked, but Serena was pretty much confirmed kill. August being a magic king might mean he has a way of transporting Serena into a separate space or even into himself like how the dragons were put into their DS. In which case it would make sense but in the end it is still pretty poor. Serena staying dead is best.