The dragonglass dagger

I believe she took her Valyrian steel dagger to kill the Night King and gave Sansa a different one. Arya probably had three or four more dragon glass blades hidden somewhere on her person, just in case. On the inside the episode segment, they said the Night King had to be stabbed in the heart, the exact spot where the Children shoved in the shard of dragon glass to create him. That was the only way to destroy him. Not sure how Arya knew that. Bran?