While fighting wars or quelling rebelions, you can recruit Mercenaries from local areas or remote corners of the world. Mercenaries are often the last resort in Imperator: Rome.
On the world map, mercenary units are stationed in capitals of different nations. You can also recruit such an army from the "mercenaries" tab. You will see there a list of all available armies that you can hire. Mercenary armies cannot be divided and they are always commanded by their general. On the other hand, you don't need human resources to replenish your losses - they use their own pool of recruits.
To hire an army you need military power points, but you need to maintain them as you would with standard armies. Mercenaries are always more costly than regular armies, and if you want to give up their services, you have to pay for the termination of the contract.
Mercenaries are a great solution when your armies have been defeated or decimated. This way you can quickly replenish your losses and repel the enemy. Thanks to mercenaries, you can strenghten your army in troops you don't normally have access to, e.g. war elephants. Mercenaries are a great solution for countries with a small number of recruits, because they do not use the national pool of human resources. On the other hand, you have to spend a lot of money to maintain an army of mercenaries and there is always a risk of betrayal.