Manta

Manta, the Behemoth of the Deep, was a creature that was shot with an arrow covered in three drops of a black liquid from a vial. The arrow was used in a crossbow to kill Manta, a creature ten times the size and strength of a dragon. It was believed that, like Deathwing, the Manta was unstoppable.

It was said that the black liquid was used to kill this beast 100 years ago which is about 90 years before Warcraft I.