If you cast this spell on followers of the enemy tribe (it won't affect you or your followers), those followers will spread apart and run away screaming. If you cast it next to a Guard Tower with a shaman in it, she will run out of the tower screaming.

You get this spell by worshipping the Vault of Knowledge in level 2: "Night Falls", and you can only have a maximum of 4 charges of it.

It is most useful to disrupting enemy attacks and stopping enemy preachers from converting your units, as well as forcing a group of enemy followers to exit the boat or baloon they are currently manning.

 

Also, in multiplayer games, swarm will instantly kill ghosts (except shaman ghosts).

Note that this spell still does damage, even if not that much.

 

A swarm will only enter a building if cast NEXT to it (a swarm won’t affect a building if it’s cast on it). Note that a shaman can always regain control of herself immediately by casting a spell (usually blast or a low mana cost spell). This is called scaring enemy's shaman out of a building (usually towers) and is used alot online.

In online gaming, swarms will affect allied troops and ghosts, so take that in mind.

If a swarm touches a invisible unit - affecting him - he will lose his invisibility and get affected. This also works against spies - they will no longer be cloaked and will panic.