Travel to Blackrock Spire and slay General Drakkisath. Gather his blood and return it to Rexxar.
You will pay the General a visit, yes, but not as one of the Black Dragonflight.
You see, a ceremony is merely another term for blood letting to the Black Flight.
The latent amulet merely needs the blood of the General in order to become active. One of their crude failsafe mechanisms.
Return to Blackrock Spire and destroy Drakkisath. Bring his blood back here and I shall activate the key to Onyxia's lair.