Mercenaries are neutral units which can be hired by most factions.
Scum, Thieves, Rogues, or groups of individuals that simply don't affiliate themselves with a single government, mercenaries come in all manner of shapes, sizes, and abilities. All mercenaries operate on their own, but can be hired by certain factions for either a fixed price or monthly payments for their services. Mercenaries can be invaluable in that they generally are able to fill soldier roles that your faction may be weak in, such as Zuna Crossbowmen filling the Darknut gap in archers.
Mercenaries differ from regular units in several ways: they are recruited outside of settlements; they have no recruitment time and can be hired as soon as the player has enough rupees to pay for them, and each mercenary type is only available in certain regions. Mercenaries usually cost more rupees to recruit and pay upkeep than equivalent non-mercenary units.
The Darknut Legion can hire randomised mercenaries in the Foreign Quarters buildable in a hamlet.