Loyalty's Mantle is a Unique Helm in Diablo IV Season 15 for Spiritborn. It comes from Horadric Cube: Upgrade Common Helm to Unique; Bartuc, Lord of Chaos - one guide / another guide.
Unique power
While your Spirit Hall choices match: Their bonuses are 100% more potent. Skills of their Base Spirit gain [50 - 60]% Vigor Cost Reduction. Skills of their Base Spirit gain [50 - 60]% Cooldown Reduction.