Forum:Voldemort's Secret Weapon in Book 5

In the OOTP, Harry stays at Grimmauld Place before he goes to school. While he's there, he Ron, and Hermione want to know what the hacket is going on. Finally, Sirius tells Harry some information about what's been going on. He tells him that Voldemort is making a powerful weapon, even more powerful than Aveda Kedarva. Maybe I just missed something, but what is the weapon, and did they explain it concretely or was there a subtle explanation that I missed?