he has to use powers from the Summer Court and the SotC
The powers of the Spirit of the Century?
(Sorry, couldn't resist)
On a more on topic note: Why shouldn't the Summer Knight be able to pick up the Sword of the Cross and go AWOL on the Summer Queens?
After all, they won't be able to get the mantle of Summer Knight back until he dies. Sure, they can compel him, IF he incurrs debt.
But sending fae after him to kill him is a bad idea because of the pointy iron stick he now wields.
Incidentally, in my game, the Winter Queens are in a similar bind. Their Winter Knight went AWOL after picking up a blackened denarius.