Quackthulhu

 is a gigantic mutated duck created by Matthew Mercer.

Appearance
Quackthulhu is a massive, swollen duck-like creature of green feathers and flesh, burning red eyes, and 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 actions
 * Each tentacle can attack or attempt a grapple each turn
 * Legendary Action: ground slam knocks opponents failing a strength save prone
 * Soul-Wrenching Quack: paralyzes opponents failing a wisdom saving throw
 * Ability to charm/take control of Lionel, possibly through his duck totem connection