D&D 5E Unusual Siege Weapons

[Imaculata]

In my pirate campaign I have a faction of pirate witches called The Speakers. They use catapults on their ships to launch necromancy-infused projectiles, which can spread disease, infest the ship with vermin, or raise the dead on an enemy ship.

I've also looked a lot at historical siege weapons, specifically those of ancient China. I particularly liked this triple crossbow/ballista, which had a huge range:

 

[Hexmage-EN]

I'd think a Hellish siege weapon would need to include biting flies and/or locusts. Maybe a hollow lance that penetrates walls and releases them?