Quackthulhu

 is a gigantic mutated duck created by Matthew Mercer.

Appearance
Quackthulhu is a massive, swollen duck-like creature of green feathers and flesh, with four whipping tentacles protruding from its body. A glowing stone that fell as a shooting star is embedded within its back.

Abilities

 * Legendary resistances (3x per encounter) and attacks
 * Ability to charm/take control of opponents
 * Soul-Wrenching Quack - paralyzes opponents failing a wisdom saving throw