Embrittle

After you hit with an attack, you can use your reaction to attempt to embrittle the target, bypassing much of their defense. If the target is a creature, they must succeed on a Constitution saving throw or become embrittled for the duration. If the target is wearing armor, under the effects of the mage armor spell or similar magic, or has Natural Armor, their armor is embrittled instead. A Large or larger creature has advantage on this saving throw. If the target is an object, a portion of the object the size of a 5 foot cube is embrittled. The spell ends at the end of your next turn, or if the target takes damage from an attack.

Embrittled Armor. Embrittled armor ceases to provide its Armor Class modification for the duration of the spell.

Embrittled Creature. An embrittled creature's base Armor Class is 1. Bonuses to Armor Class like Dexterity, the shield spell, or the Unarmored Defense feature of barbarians and monks apply as normal. 

As an example, a barbarian with a +1 to Dexterity and a +3 to Constitution that isn't wearing armor would have an Armor Class of 5 while embrittled (1 base AC + 1 + 3 = 5). A wizard with a +1 to Dexterity and benefiting from the mage armor spell would have an Armor Class of 11 while the armor was embrittled (10 base AC + 1 = 11).

Embrittled Object. An embrittled object's Armor Class is 1, and is vulnerable to bludgeoning damage.