Similar to player company there are other mercenary units that take contracts for the living in Darklands.
They are highly dangerous, and confrontation should be avoided, unless the company is up for a challenging fight with great loot. Also notice, that you will have a negative opinion penalty with a town or house, which hired a mercenary band if you dicide to attack one. This doesn't concern an event when delivering a cargo contract, in which you can also fight them, but will not have opinion penalty with anyone (and will not know who hired them or whether they acted independely). Anyway if you attack one mercenary band, others mercenaries will not pay attention on it - you will not have negative opinion with them.