Bandits!

[Immaculate]

Whats this event do:



Now do i root out the brigand leader? Will i have to fight a unit and its 'gang'? If so, what can i expect?

[Calahan]

The event will cause unrest to increase in the province each turn until you find and remove the problem (by patrolling usually).

Once found, the resulting fight isn't very tough, usually about 30-40 villian units IIRC. Most of the time they will route the moment you inflict any damage on them, as they will very likely have only 7 morale. Which is 8 base, and -1 from being in hostile dominion (even if your dominion is not in the province, any dominion there at all will be hostile to them)

[Quitti]

Indeed. Just get some ~20 units to patrol there and it should solve itself. There are a few other similar events/spells that do that. Monster boar (nature magic) and dream horror (blood magic) spring to mind.