To take TML's thoughts a step further, I'd like to quote a section from this last chapter:
"I have found the owner of Slifer," sighed Twilight softly.
The bird paused and landed besides her. Slifer has been found? Who owns it?
Twilight explained her visions, and Sibylla was as stunned as a bird could look.
If...what you say is true, Miremei, then this entire area is in danger! Slifer is one of the more terrifying of beasts. Just as Obelisk contained my fury, so too did Slifer contain my fire. We must confiscate it.
"I cannot risk stealing it from him before the duel, or else I will lose my job," whispered Twilight.
Maybe it's just me, but I thought the fact that the visions with Slifer were connected with the upcoming duel kind of gave it away. But perhaps others saw it differently. I dunno. *shrugs*