Quicksilver Sword (Patreon)

Weapon (longsword or shortsword), very rare (requires attunement)

You gain a +2 bonus to attack and damage rolls made with this magic weapon, which has the finesse property. As the blade moves, its reflective surface shimmers like water on a mirror.

The longsword deals an extra 1d6 poison damage to any creature hit by it. It transforms into a shortsword that doesn't deal extra poison damage as part of using either of the named properties below. Once a property has been used, it can't be used again until the next dawn.

Silver Sickness. When you hit a creature with the longsword, you can choose to transform the weapon into its shortsword form and leave some of its metal inside the wounded target. The creature must make a DC 16 Constitution saving throw. On a failed save, it takes 3d6 poison damage and is poisoned for 1 minute. On a success, the creature takes half as much poison damage and isn't poisoned. While poisoned, a creature takes 1d6 poison damage at the start of each of its turns. At the end of each of its turns, the creature can repeat the saving throw, ending the effect and expelling the leftover metal on a success.

Toxic Armor. You can use a bonus action to allow yourself to be poisoned by the blade instead, transforming the weapon into its shortsword form in the process. You remain poisoned for 1 minute or until you end the effect early (no action required). While poisoned, you have resistance to bludgeoning, piercing, and slashing damage, as the metal within your body helps defend it against attacks.

When a creature is no longer poisoned by one of these properties, the weapon's metal is expelled from the creature and returns to the blade, transforming it back into its longsword form.

As deadly within as it is without.