Ring of Armored Strength (5e Equipment)
From D&D Wiki
Ring, very rare (requires attunement)
This small silver ring was said to be created by a wizard who made his robes of malle, despite lacking the strength for it. Hindered by his armor too often, he created this ring to ease the burden.
While worn, it grants the wearer proficiency in medium and heavy armor, and sets the wearer's Strength to 15.