The Aberrant Dragonmark feat from Ebberon, Wayfinder’s Guide to Eberron and in Eberron: Rising from the Last War says:
At the DM’s option, a character who has the Aberrant Dragonmark feat has a chance of manifesting greater power. Upon reaching 10th level, such a character has a 10 percent chance of gaining an epic boon from among the options in chapter 7 of the Dungeon Master’s Guide. If the character fails to gain a boon, they have a 10 percent chance the next time they gain a level.
What are some rules from sources other than Chapter 7 of the DMG, that might prompt a DM to give players a boon or supernatural gift, from official WotC sources only, excluding UA?