Agathys armor

A protective magical force surrounds you, granting you temporary hit points.

At level 1: You gain 5 temporary hit points.

At higher levels: The spell’s effects improve. When cast using a spell slot of 2nd level or higher, the temporary hit points increase by 5 for each slot level above 1st.

When a creature hits you with a melee attack, the creature takes cold damage equal to the temporary hit points remaining on you.

Flavor for Riku:

The protective force manifests as dark, shadowy frost, a physical embodiment of Riku's bond with Kurogane. The cold damage reflects the psychic strain of the pact, as the very essence of Kurogane lashes out at those who strike him.