Anything Swords and Anything Items
Advanced Dungeons & Dragon’s Unearthed Arcana has an interesting magical item that I have never seen used: the Anything Sword.

As with several other things in Unearthed Arcana, this power comes with restrictions that make it almost not worth having. Ahem. 😀
This item has great potential for roleplay. So I am going to update it for use with D&D 5e. The biggest change I am making is removing the impermanence.
Anything Sword
Weapon (any sword), Legendary
This sword has a base type (e.g. short sword, longsword, et. al.) and a base +1 magical bonus. While drawn and held, you can use an action to have it transform into another magic sword. This is limited by the rarity of the new form:
- A Uncommon sword, any number of times per day
- A Rare sword, once per day (resetting at dawn)
- A Very Rare or Legendary sword, once per day (resetting at dawn)
It may stay in its new form indefinitely. If still in a Rare, Very Rare, or Legendary form at dawn, it remains in that form but it also expends the appropriate usage for the new day. Note that the sword type counts as part of its form. For example, it cannot change from a Vorpal longsword to a Vorpal greatsword as the latter would be a second Legendary form.
This can be taken further.
Anything Item
Wondrous Item, Legendary
This item has the base form of any Uncommon magical item. While held, you can use an action to have it transform into another magical item. This is limited by the rarity of the new form:
- A Uncommon item, any number of times per day
- A Rare item, once per day (resetting at dawn)
- A Very Rare or Legendary item, once per day (resetting at dawn)
It may stay in its new form indefinitely. If still in a Rare, Very Rare, or Legendary form at dawn, it remains in that form but it also expends the appropriate usage for the new day. Note that the item type/size counts as part of its form just like the Anything Sword.
So it can be all magical items but only one at a time, and only two big ones a day. Very powerful but I do not believe abusively so. Properly legendary.
FYI, the Elemental Blade of Fire is an Anything Item. Its base form is a Ring of Warmth. 😀