"The xeno was there, standing in a doorway. It looked much like the depiction on Benz's wall: a cross between protoss and zerg anatomy, with a thin face, an expansive carapace headpiece, segmented plates over spindly limbs,
and massive black claws. It was towering and hulking, a dominating presence, strange and unique and wholly alien. And its eyes...its eyes reminded Dorian very much of the relic's black surface. A vast, immeasurable void waited behind those orbs, and Dorian felt himself falling into them, becoming lost."
- Dorian's thoughts upon encountering a hybrid reaver(src)
Raynor's Raiders discovered the creature and destroyed its holding cell. When the cell was destroyed, the reaver then proceeded to destroy the facility. The Raiders barely escaped into a Hercules, after which the reaver "screamed" at them.
The following section contains information from a previous version of StarCraft II which is no longer valid.
Originally the protoss-based hybrid destroyer was the only type of hybrid unit in StarCraft II. The idea of a zerg-based hybrid had been pitched by Samwise Didier in concept art, (labelled as a "high breed") but was effectively abandoned for a year. Eventually the idea of a zerg-based hybrid was pitched within the design team and Didier's concept was resurrected as the hybrid viper. In the final game it was known as the hybrid reaver—mostly the same, but unlike the viper, it had tentacles coming off its back, that aren't visible on lower graphic settings (the player must set "effects" to medium or above).