Mercenaries are NPCs you can encounter in mercenary buildings. They spawn on top of their respective blocks, then propose to be hired for some ressources. By default, they’ll follow you for one gold ingot or ten iron ingots. If you hold in hand one of these ressources in front of them, they’ll tell you the amount they need. Simply right click on the mercenary with the appropriate payment.
Once you’ll pay them, they start following you. But don’t worry! It won’t prevent you from having a private life! You only have to right click on a mercenary so he stops walking. Click once again and he’ll follow you again.
If you chose to attack a mercenary instead of hiring him, be carefull: he will fight back!
There are three different mercenaries: knights who fights with a sword, archers who throw arrows, and healers who makes you recover health.