Source: Bigby Presents - Glory of the Giants

Ring, Very Rare (Requires Attunement)

This ring is carved from hematite and bears an engraving of the friend rune.

When you first attune to this ring, you can touch one willing creature and form a magical bond between the two of you. While this bond lasts, whenever you are subjected to a spell or magical effect that restores hit points, the bonded creature also receives the benefits of the spell or effect.

You can bond with a different creature whenever you finish a long rest, provided that you can touch the creature and the creature is willing.

A creature can benefit from only one ring of amity’s bond at a time. The bond ends if either you or the creature travels to a different plane of existence, if you bond with a different creature at the end of a long rest, or if you sever the bond as a bonus action.

Invoking the Rune. When the bonded creature hits a target with an attack roll, you can use your reaction to invoke the ring’s rune if you are within 60 feet of the bonded creature. The bonded creature’s attack is then turned into a critical hit.

Once the rune has been invoked, it can’t be invoked again until the next dawn.