>>118808452They agreed to the fight because the land that Arakko (the invading cousin of Krakoa) is passing through to get to Krakoa and Earth is the land that Saturnyne runs. She's powerful enough to stop both forces, she literally turns Death into a shota child to demonstrate this.
So, in turn, she decides that instead of having an all-out war on her front lawn, it'll be a contest of champions wherein 10 v 10 swords clash.
As for your second question, Saturnyne is powerful over her own land (which is the only path available for Arakko to get to Earth) so she calls the shots
