Multiattack

Arbeyach makes one bite attack and two claw attacks.

Bite

Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 15 (2d8 + 6) piercing damage plus 9 (2d8) poison damage. If the target is a creature, it must succeed on a DC 22 Constitution saving throw or be cursed with Arbeyach rot. The cursed target is poisoned, can't regain hit points, its hit point maximum decreases by 13 (3d8) for every 24 hours that elapse, and vermin attack the creature on sight. If the curse reduces the target's hit point maximum to 0, the target dies and immediately transforms into a randomly chosen swarm of insects. The curse lasts until removed by the remove curse spell or comparable magic.

Claw

Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 10 (1d8 + 6) slashing damage plus 9 (2d8) poison damage.

Vermin Breath (Recharge 5-6)

Arbeyach exhales vermin in a 120-foot line that's 10 feet wide. Each creature in the line takes 54 (12d8) poison damage, or half damage with a successful DC 22 Dexterity saving throw. Each creature that fails this saving throw must succeed on a DC 22 Constitution saving throw or be cursed with Arbeyach rot (see the Bite attack). In addition, Arbeyach summons a swarm of insects (of any type) at any point of the line. The swarm remains until destroyed, until Arbeyach dismisses it as a bonus action, or for 2 minutes. No more than five swarms of insects can be summoned at the same time.