My vote is monk from age of empires 2. These dudes can walk up and stare at your friend chanting for a bit, then suddenly they turn around and attack you. That’s horrifying.
Wololo
Basic builder units. You commit genocide against an entire civilization but leave one of these fuckers alive and 20 minutes later you’re facing en entire army.
I’d prefer a commander from Supreme Commander. It can raise an entire military by itself, and it’s much hardier than basic builders. It’s deadly up close. Even if you somehow defeat it, your reward is a nuclear explosion.
I remember the matches where you transport drop you commander into the enemy base, so both your and their commander blow up inside their base. The hardest part was getting the game rules past the eyes of your friends while setting up the game.
An ultralisk was the first thing that I thought of.
Titan from age of mythology. That one guy can destroy your whole base.
Dark Templar from StarCraft. Just going about your business then suddenly you’re sliced in half.
A Ghost dropping a nuke is pretty scary but not much different from the real life possibility of dying from a mile you didn’t know was coming. Just not from an invisible person.
Being infested by the zerg would be pretty terrifying too. Not really a specific unit though, I don’t think. Haven’t played in quite a while.