Mercenaries are unlocked when you beat zone 139 or ascend for the first time. You'll unlock your first mercenary after a 8-hour quest, and after that you can send them off to do various tasks (find gold, find hero souls, find a relic, find a ruby, or activate a random skill). Occasionally, they will fail their task and die, and you have the choice to revive them or bury them. Choose 'bury', and then either do the 8-hour quest to get another mercenary, or have another mercenary do an 8-hour quest to find another mercenary.
When this title update hit, I already had 5 mercenaries, so I just kept burying one whenever he died, and had another one go on the quest to get another mercenary.