Your equipped Relic assigns the passive ability and perks for your primal transformation.
Unlike Power, Relic abilities are triggered automatically when certain conditions are met. Relic abilities do not necessarily affect only the character using it, some Relic grants positive buffs to nearby allies or even negative buffs to nearby enemies.
Feral Inspiration, which only Sunder can equip, is the default Relic selected when you’re just starting the game.