What was the weapon in 'Harry Potter and the Order of the Phoenix'?

Answer 1

a prophecy that professor Trelawney made

(There was something about an extreme powerful weapon, but finally nothing came out of it.)

something came out of the prophecy,but no one noticed it because they were fighting but Dumbledore did.15years ago.it was what professer Trelawney said.

"the boy that was born when July dies away."and "niether of them lives,when the others is alive."

Answer 2

It isn't a weapon, it was the prophecy. They just presumed it was a weapon.