Rift Sheath (Patreon)

Weapon (quarterstaff or shortsword), rare (requires attunement)

This dark, wooden staff is actually a two-sided sheath for a pair of equally dark shortswords. While holding the staff with two hands, you can use a bonus action to twist the staff to sheath or unsheath the two swords from it. When unsheathing the swords, the wooden sheath is shunted into a special extradimensional space until you decide to sheath the swords again, at which point it magically reappears between the swords.

The weapon has the following properties depending on whether the swords are sheathed as a quarterstaff or unsheathed as a pair of shortswords.

Quarterstaff. While wielded as a quarterstaff, you gain a +1 bonus to attack and damage rolls made with this magic weapon, which has the finesse property. While holding it, you can use a bonus action to speak the staff's command word to create a spatial rift around you that lasts as long as you concentrate (as if concentrating on a spell), up to 1 minute. For the duration of the rift, other creatures within 15 feet of you must spend 2 feet of movement for every 1 foot they move. Once this property has been used, it can't be used again until the next dawn.

Shortswords. These magic shortswords have the thrown property with a normal range of 15 feet and a long range of 30 feet. Hit or miss, the shortsword vanishes in a mirage-like distortion and immediately reappears in your hand after the attack. When you hit with a ranged attack using one of the swords, you can use your reaction to teleport to an unoccupied space that you can see within 5 feet of the target. When you do, you have advantage on the next attack roll you make against that target before the end of your next turn. Once this property has been used, it can't be used again until 1 hour has passed.

The space between is always there: you just need to look for it. What's to say that we're in the rift, and the real world is out there?