The Romans were very skilled in a specific area. Animal warfare. Among all other advancements, this was the most terrifying one. They fed prisoners and criminals to lions, tigers, lampreys, even herbivores like bulls. Anything they could get their hands on. There are many examples of animals being used to incite terror and death in one's enemies.

Head crabs: Terrible creatures from Xen that can wipe out a settlement in a few hours, used by The Combine as a bio weapon against rebels, until it backfired horrendously. these little monsters can latch  onto your head and burrow their 'beak' into your skull, turning you into a zombie under it's control, while you are still alive.

Chryssalid: A nightmarish, insectoid creature, used as a mindless terror weapon by the Ethereal Collective. The are almost impossibly fast, have razor sharp claws and pincers, and turn their victims into zombie hosts for their offspring.

\A Hunter: Origin and species unknown, Can't tell if it is organic or cybernetic, armed with razor sharp fangs, powerful legs that can crush your skull, and energy weaponry implanted on it's head. Used by the combine in order to kill even the most unkill-able of people.


A Xenomorph, the deadliest thing we know of. They were created by some civilization as a bio weapon. That culture is gone, but these remain. You can't hide from it, can't outrun, and is immensely hard to kill, even with high powered weaponry. the deadliest weapon, their 'workers' find a host and implant an embryo, and 48 hours later a full grown Xenomorph violently bursts from the hosts chest.