Kostas kills Edelgard
There
No more Flame Emperor
Alternatively
Kostas kills Dimitri. Claude can’t hold Leceister on his own without the Kingdom to tie half the Empire down, meaning he’d just bail to Almyra and Fodlan would be unified much faster.
If Kostas kills Claude, then it would be CF except Byleth doesn’t need to join Edelgard, due to the lack of an Alliance making the Kingdom-Empire battle very one-sided and swift.