Two Heads. The Adder has advantage on Wisdom (Perception) checks and on saving throws against being blinded, charmed, deafened, frightened, stunned and knocked unconscious.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) piercing damage. 

Quills. Melee Weapon Attack: +5 to hit, reach 5ft., one target. Hit: 8 (1d10 + 3) piercing damage. The target must succeed on a DC 12 constitution saving throw or is poisoned until the end of its next turn.

Adder's Fang. The Adder fires a poison dart at a target within 60ft. of itself that it can see. The creature must succeed on a DC 12 dexterity saving throw or is hit by the poison dart and suffers 3 (1d6) piercing damage. The creature takes 5 (2d4) poison damage at the beginning of each of its turns for one minute or until a creature spends an action to remove the dart from its body.

Molt (1/Long Rest). The Adder sheds its skin a regains 9 (2d8) hit points. It also ends any lasting effect or condition of non-magical origin that is affecting the Adder, such as Alchemist's Fire or poison.