Boon of Class Mastery (5e Epic Boon)
From D&D Wiki
Boon of Class Mastery
Once you have reached level 20 in a single class you may take this boon to gain a feature from a different subclass within the same class. When taking this boon you gain all of the features from that subclass. You can acquire this boon more than once. If you do, than you can choose a new subclass and gain it’s features as well.
Example: A Totem Barbarian wants to take the features from the storm path so they take this boon and gain the first subclass feature. However, since they are already a totem barbarian they may not take a new first level totem.