Blade of the Ashen Hills (5e Equipment)
From D&D Wiki
Weapon (Katana), Legendary (attunement required)
This long, two handed ōdachi was forged by a once honourable warrior who broke his oath of mercy by killing an unarmed, surrendered opponent. For this his deity cursed him to be felled by his own blade, then to have his soul bound to the blade. When he did die, by falling on his own blade in combat, his soul violently exited his body in a fireball. Which charred his sword as it entered, giving the blade a smokey black colour and turning the red hilt wrapping burnt maroon. Frightened of what happened, others quickly sheathed the blade and tossed it into a volcano. However, over centuries it merely found its way into a mountain cave.
On a normal attack the blade deals 1d12 slashing, doing an additional 1d8 fire damage on a critical. However, the user can sacrifice parts of their soul to the blade to enhance their strikes. By gaining an additional level of madness the wielder can deal an extra 1d6 slashing and 1d4 fire damage. Additionally, after prolonged exposure the soul becomes corrupted anyway. After either 10 successful attacks or 1 hour with the sword unsheathed the user gains one level of madness.