Your blood rushes to harden your skin, forcing you to take 1d4 necrotic damage. Attacks against you have disadvantage for the duration.

The amount of necrotic damage needed to protect yourself against harm increases by 1d4, and your AC also increases when you reach 5th level (2d4) (1), 11th level (3d4) (2), and 17th level (4d4) (3).