Oh my god, everyone became fucking mentally retarded this arc. Not a one of them can speak plainly so instead they engage in combat through and above the royal capital.
The reason Kururi's sword can command one dragon, and not the other, is that different species of dragon have a hierarchy. The one he commandeered is below the life dragon the sword's made of, the one Rasha is riding is above it.
This feels so incredibly forced. Author have completely sacrificed the intellect of the the characters, to the point of retardation, on the altar of extremely cheap drama.