Hidden Blade (5e Equipment)
From D&D Wiki
Martial Melee Weapons
A small blade, mounted to the forearm. Thanks to the mechanism covering the wrist-mount, the blade is capable of freely extending and releasing itself, but it is still small enough to be well disguised under a cloak.
Special. When you make an attack against a creature that has not yet taken a turn since initiative began, they take 1d4 additional damage.