Murasame, The Demon Blade (5e Equipment)
From D&D Wiki
Weapon (Katana), Legendary (requires attunement by a chaotic character)
An ancient katana imbued with the power of a demon lord. If a creature who is not chaotic tries to attune to this weapon, they take 10d6 poison damage and drop the weapon.
Demonic harakiri. The wielder of this ancient weapon can take an action to cut themself, causing them to take 1d8 slashing damage. They will be restrained until the start of their next turn as their body begins to change. For 1 minute, their eyes turn blood red, their skin becomes darker, abyssal letters begin appearing on their skin, and their veins grow as they pump more blood. On tieflings it triggers their demonic ancient bond, their horns grows, their nails becomes claws and their voice changes to a much much darker one.
Any creature in this form gains the following:
At the end of each of the users turns in this form, they have to roll a DC 10 Constitution saving throw which increases by 1 on each success. On a fail, the user is rendered unconscious for 10 minutes, unless greater restoration is used.
If the effect lasts more than 3 turns, the weather changes to soft rain. After 2 or more turns, the rain becomes heavy.
When the wielder return to their normal form. the rain stops and for some days they will have scars where the abyssal runes appeared.