Magical Resistance. Advantage on saving throws against magical effects

Abyssal Corruption. Whenever an attack deals necrotic damage to a target, the target must make a dc 17 Constitution save. On a failed save, their maximum hit points are reduced by a total number and roll on the flesh warping table. This change lasts until a long rest is successfully completed. 

Black Claw. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 22 (3d8 + 8) piercing damage. and 2d10 necrotic damage

Ichor Bolt. Ranged Weapon Attack: +7 to hit, range 20/60 ft., one target. Hit: 24 (2d10 + 8) Necrotic damage. 

Multiattack: This creature makes two attacks, either two claw attacks, two ichor bolts, or one of each. 

Enter the description for how mythic actions work for your monster here.

Tendrils of Baphomet (Costs 2 Actions). Yrel stretches his arms out and inky black tendrils explode out from his location in a 30 foot radius. Any creature caught in the radius takes 5d12 necrotic damage and must succeed on a dc 17 constitution save or be immobilized.