Source: The Book of Many Things

Weapon (warhammer), very rare (requires attunement)

This magic hammer has 3 charges. As a bonus action, you can expend 1 charge and slam this hammer on the ground, creating a 15-foot-radius circle of glowing runes centered on the point of impact. While you’re inside that area, your hammer glows with matching runes, and you gain a +3 bonus to attack and damage rolls made with this hammer. The rune circle disappears after 1 minute, when you create another rune circle, or when you dismiss the rune circle as a bonus action. This hammer regains 1d3 expended charges daily at dawn.