Source: The Book of Many Things

Weapon (any sword), legendary (requires attunement)

You gain a +3 bonus to attack and damage rolls made with this magic sword.

This sword can tear the fabric of reality, creating a temporary rift between planes. You can use your action to choose a different plane of existence from the one you’re on and slice through an unoccupied space within 5 feet of yourself, creating a rift to that other plane. The rift can be up to 10 feet high and 10 feet wide, and it lasts for 1 minute. Once this property is used, it can’t be used again until the next dawn.

You can specify a target destination, such as the City of Brass on the Elemental Plane of Fire or the palace of Dispater on the second layer of the Nine Hells, and the rift opens in or near that destination (DM’s discretion). If you are trying to reach the City of Brass, for example, the rift might appear on the Street of Steel, before the Gate of Ashes, or facing the city from across the Sea of Fire, at the DM’s discretion.

Anything that enters the rift is instantly transported to the other plane, appearing in the unoccupied space nearest to the rift.