Vampires drink the blood of a clown.

They drop dead on sight. Everyone knows the clown is a poisonous thing to all beings in existence, so why should its blood be any different? You don't need garlic or a stake in the heart to repel these creatures. All you need is the clown to keep bleeding. Its blood can annihilate all your enemies with one drop. One drop is enough to slaughter an army. Airplanes will pour the blood of a clown on populations during war.