After you unlock Mercenaries (see achievement guide linked below), you can have them do quests: find gold, find relics, find hero souls, find rubies, or activate a random skill. (There are no achievements for activating skills.) The quest durations vary from 5 minutes to several days, and have varying chances of success. It's best to combine this with the achievements for completing x quests lasting y minutes (like 'Short Work'). Simply complete 100 Ruby quests successfully and you will unlock this.