Mercenaries cost mythium and are used to attack your opponent. Mythium can be spent on mercenaries every wave or saved for multiple waves to unleash a larger attack.
Mercenaries attack your opponent whenever the next battle phase begins.
Mercenaries hired during the build phase will attack your opponent as soon as the battle phase begins.
Mercenaries hired during the battle phase will attack your opponent when the next wave's battle phase begins.
Each player attacks exactly one other player, depending on which lane you are in.