Character "Signature Weapon"

[Lucan Aligheri]

My group is playing a level 20 campaign, and the whole point of what we're doing is to make our characters as stupidly overpowered as possible (The DM scales the monsters). We also each get a signature weapon, akin to Excalibur, or Mjolnir, or Captain America's Heater Shield, a weapon that is a staple of a character. We get to design our weapons via a wish, and the DM promises not to screw with it as long as it isn't game breaking. I want a weapon that is very similar to the extending spear of Prince Nuada from Hellboy II, and I want it to have multiple function: A short baton form, a handaxe form, a long staff form, and a glaive form, and I'm trying to find out if there are any weapons already in D&D that are similar to this design, so that I don't have to think up of nerfs and buffs that could potentially be exploited later.

[Daishain]

Isn't there a rod with functions like that? hold on

EDIT: Ah yes, try checking out the Rod of Lordly Might, a device with multiple buttons that changes its form. It is a legendary item, and thus considered incredibly powerful, but that apparently isn't an issue for the campaign.

[Raxxius]

Daishan has the best standard option on the table.

The other obvious option is to play a Hexblade - which will allow you to change your weapon to whatever you need.

[Slipperychicken]

Back in the day, we had morphic weapons. Those could change once per round into a weapon of your choosing (i.e. go from sword, to spear, to axe, etc.) while still retaining the other enchantments. It's not a terribly strong enchantment, since you could get a similar effect by just carrying around the different weapon types. As a result, I think you could pretty safely put it on a homebrew weapon in 5e.