BARKSKIN - Transmutation - 2nd Level

Casting Time: 1 Action
Range: Touch
Components: V, S, M (a handful of oak bark)
Duration: Concentration, up to 1 hour

  You touch a willing creature.  Until the spell ends, the target's skin has a rough, bark-like appearance, and the target's AC can't be less than 16, regardless of what kind of armor it is wearing.

Source: Player's Handbook, 5th Edition SRD
Class List: Druid, Ranger.