User blog comment:Omega natsu2/Chapter 371: Tartarus Arc, Part 2: Song Of The Heavenly Dragon, Predictions./@comment-86.70.179.7-20140210084523/@comment-7418318-20140210214946

It's a silver key, so I doubt it'd summon something overwhelmingly powerful.

As I said before it's probably just a Humanoid in a Dragon costume with some fire magic on part with Macao or somesuch.