What are the most terrifying weapons that ancient civilizations used?

I’d say burning liquids launched by catapult or trebuchet. Have some boiling oil or a potful of heated tree pitch dumped on a squadron of enemy, take them right out of action. And getting hot pine tar thrown in your face will disfigure you for life if it doesn’t kill you.

Mace and chain with morning star. Or the mace, minus chain, that was known as a morning star.
Pitchfork. Farm tools, such as the flail, were often used in ancient combat.

Greek fire was an incendiary weapon used by the Byzantine Empire beginning c. 672. Used to set light to enemy ships, it consisted of a combustible compound emitted by a flame-throwing weapon. Some historians believe it could be ignited on contact with water, and was probably based on naphtha and quicklime. – - Wikipedia

Not an open field weapon, but a weapon for torture: The Iron Maiden.

They catapulted diseased animal corpses over castle walls to spread disease: https://en.wikipedia.org/wiki/History_of_biological_warfare