Adder of Strife (5e Equipment)
From D&D Wiki
This item first appears to be a 20cm golden snake with emerald eyes that wraps around your forearm. When activated as a bonus action, it uncoils and slithers into your hand in the form of a golden spear or dagger. The transformation lasts until you deactivate it as a bonus action. Your attacks with this weapon count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.
On a critical hit with this weapon, it unleashes a wave of screams which deals 4d8 necrotic damage (which isn't doubled with a critical hit) to all creatures within 15 feet of the target. This extra damage does not affect creatures that are deafened, those that can not hear, constructs, or undead.