Worldeater, the Last Blade

Curving gently along the four foot length of its blade, this longsword is a work of unparalleled craftsmanship. Its hilt, which makes up the remaining foot and a half of its length, wrapped in crisscrossing silk bands in alternating black, red, and yellow, suggests it is meant to be wiedled with two hands, yet the weapon is so light that it can easily be wielded with one.

The edge, polished to a shine, is razor sharp and thrums with magic. Even unattuned it easily outstrips a mundane weapon in power. Who could say what might it hides, what secret techniques lead to its creation, or what terrible violence has transpired about it?

And how did it come to be here, so far from any battlefield?

DM's Guide

When your players first encounter this weapon, describe it as above, and tell them it has the following attributes:

Mysterious Sword

Bonus Damage Features +1 1d8 Slashing Versatile (1d10), Finesse

You can describe it as "A +1 Longsword with Finesse. It can be attuned, potentially unlocking further benefits."

Should a player attune to the weapon, it reveals its true nature as a cursed weapon.

This is an item that requires some leniency on the part of the DM. Run as strictly as possible, it is very easily to effectively kill the player wielding the weapon, or wipe the entire party. Try to be more interesting than that. It's a roleplaying game, so roleplay. Worldeater is in many ways a character in and of itself, and can be nearly as powerful as an entire additional party member.

Worldeater

Legendary Longsword, Artifact

Carrying an ominous moniker reflective of its abilities, Worldeater is a cursed weapon, possessing a malevolent intelligence and an unending thirst for violence and destruction.

Abilities

Worldeater, the Last Blade, has the following abilities. Some abilities refer to a "wielder". This is a character who has attuned to the weapon. Characters who wield the weapon but have not attuned to it are not wielders.

Invincible

So deep run its curses and resentment, Worldeater can never be truly destroyed. Should it be damaged but not annihilated outright, the sword can be regenerated by its attuned wielder as a bonus action. If not regenerated in this way, it will regenerate more slowly, over the course of a day.

Should the blade be utterly destroyed, likely through extraordinary means, it is not gone forever. It will slowly regrow over the course of a month, sprouting out of the earth in some dark corner of the material plane. Anyone attuned to the blade will know the distance and direction to the reforming blade as soon as this process has begun, as long as they are on the same plane.

Worldeater is as resilient as other magical weapons to physical harm.

Intelligent Weapon

Worldeater is self-aware and keenly intelligent. It is capable of observing the world around it as with Blindsense out to 30 feet, and can telepathically communicate with any wielder it has. Its mental statistics are Intelligence 18, Wisdom 18, Charisma 20.

The Last Blade

Once a character attunes to Worldeater, the attunement becomes permanent. They can no longer attune to other magical items or unattune for Worldeater, and any other magical items to which they are attuned become immediately un-attuned. Worldeater can be unattuned only via a Remove Curse spell cast at at least half the character's current level or challenge rating, rounded down, cast on the weapon, and should Worldeater not be in the character's possession the curse cannot be removed save through divine intervention(1). While a character is attuned to Worldeater, they cannot willingly allow it to leave their possession, and should it be taken away from them feel an urge to do everything in their power to retrieve it, though they do not ahve to act on this urge. So long as they are attuned to Worldeater, they know the exact distance and direction to its current location, so long as it is on the same plane.

Any number of creatures can be attuned to (and cursed by) the blade at once, all of them knowing of the weapon's location and unable to attune to magic items.

(1) 20th level characters or creatures with a CR of 20 or higher can only unattune from Worldeater through the effect of the Wish spell or a similarly powerful form of magic.

Mental Fortress

Though intelligent, Worldeater is immune to any effect which would control or alter its conciousness. In effect, it always automatically succeeds on mental ability saves.

Additionally, while its wielder is unconcious and within 10 feet of it, such as while they are asleep, Worldeater can draw their mind into itself, creating a mental construct in the form of a white void. In this space Worldeater can appear in any form it chooses, and can freely shift the environment to its liking. It can also converse with its wielder person-to-person, though only in a fictional setting.

Once it has presented a form to the wielder at least once, Worldeater can cause the wielder to hallucinate this form being projected into the world around it. The wielder is aware this form is unreal, but can converse with it, and Worldeater can speak through it in kind.