This sword is a shadowy, incorporeal scimitar that damages the mind and the soul. Upon attuning to this weapon, one must succeed on a DC 15 Sanity Saving Throw, or gain an effect of Short-Term Madness and take 14(4d6) damage, which is both necrotic and psychic. On a success, the creature gains an effect of Long-term madness. If the sanity ability score is not in play, use Wisdom instead.

Curse. Whenever you roll a critical fail while using this weapon, you must make a DC 15 Sanity Saving Throw or gain another effect of Long-Term Madness. If you try to part with it, you must succeed on a DC 20 Sanity Saving Throw, taking 14(4d6) damage, which is both necrotic and psychic. 1d4 Crawling Horrors and 1d4 Terrorbeaks(see link in comments) also appear, which are hostile.

Proficiency with a scimitar allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Cursed, Finesse, Light