Source: Critical Role - Call of the Netherdeep

Weapon (any), very rare (requires attunement)

This magic weapon has a dull, rusty color or has veins of ruidium running through it. While this weapon is on your person, you gain the following benefits:

• You can breathe water.
• You gain a swimming speed equal to your walking speed.

Ruidium Strike. A creature you hit with this weapon takes an extra 2d6 psychic damage.

Ruidium Corruption. When you roll a 1 on an attack roll made with this weapon, you must make a DC 20 Charisma saving throw. On a failed save, you gain 1 level of exhaustion. If you are not already suffering from ruidium corruption, you become corrupted when you fail this save.

If Ruidium Is Destroyed. If the Apotheon is killed or redeemed, all the ruidium in Exandria is destroyed instantly, and a Ruidium Weapon becomes a +2 Weapon .