Source: Princes of the Apocalypse

Wondrous item, rare (requires attunement)

A Weird Tank is a ten-gallon tank of blown glass and sculpted bronze with a backpack-like carrying harness fashioned from tough leather. A water weird (see the Monster Manual for statistics) is contained within the tank. While wearing the tank, you can use an action to open it, allowing the water weird to emerge. The water weird acts immediately after you in the initiative order, and it is bound to the tank.

You can command the water weird telepathically (no action required) while you wear the tank. You can close the tank as an action only if you have first commanded the water weird to retract into it or if the water weird is dead.

If the water weird is killed, the tank loses its magical containment property until it spends at least 24 hours inside an elemental water node. When the tank is recharged, a new water weird forms inside it.

The tank has AC 15, 50 hit points, vulnerability to bludgeoning damage, and immunity to poison and psychic damage. Reducing the tank to 0 hit points destroys it and the water weird contained within it.