Hypnotise is a spell that converts some enemy followers that are in the area where the spell was casted, to your side for about 40 seconds. The spiral above the follower's head indicates that they are temporaliry hypnotised.

 

Hypnotised followers can still be converted by enemy preachers and hypnotised again to another tribe.

You first receive hypnotise permanently on Level 11: "Treacherous Souls" by worshipping the Vault of Knowledge that the Chumara tribe guards. You can store a maximum ammount of 3 shots of this spell at once.

This spell is one of the best counters of magical shield, since it can be casted directly on enemy' magic shielded units.

 

Hypnotise is not commonly used in multiplayer games, but sometimes people enjoy playing with it on.

 

When training hypnotised followers to another follower type they will get convert to your tribe permanently!

 

The Chumara tribe is popular for using the Hypnotise spell commonly throughout the game.

 

Also, if you defeat a tribe while having some of its followers hypnotised, they convert to your tribe permanently.

 

You can cast hypnotise on ghost army units, killing 6 ghosts for each hypnotise casted. Casting this spell directly on buildings will not affect enemies inside of it.