A.R.S.B.T.B.J.G.O.F.E.

Weapon (requires attunement, longsword), very rare

Also known as A Regular Sword But the Blade Just Goes On Forever, this weapon appears to be normal at first, but when a creature attuned to the blade uses its action to push on the gemstone in the middle of the hilt, the blade immediately extends to be 20 feet long, and becomes too unwieldy to use. At the start of each of the creature's turns, the blade extends another 20 feet, unless it cannot grow longer, such as if a stone wall is in the way.

The creature can use its bonus action to immediately have the blade shrink to regular size, and be wieldable once more.