Dusk-Flame Blade

Source: Spell Index of S'uorhta

You weave threads of shadow material from the Shadowfel around your blade to create an unnatural, silent black flame that causes the area within 5-feet of you to become dimly lit regardless of the current lighting. As part of the spell, you brandish your weapon and make a melee attack with it against one creature within 5 feet of you. On a hit, the target suffers the weapon attack’s normal effects and is engulfed by the blacked flames of your weapon, taking additional psychic damage equal to your spellcasting ability modifier. At the start of the creature's next turn, it must make a Constituation saving or take an additional 1d8 necrotic damage.

At Higher Levels. At 5th level, the melee attack deals an extra 1d8 psychic damage to the target on a hit, and the damage the target takes for failing its Constitution saving throw increases to 2d8. Both damage rolls increase by 1d8 at 11th level (2d8 and 3d8) and again at 17th level (3d8 and 4d8).

Insight of S'uorhta: Derived from the 'Green-Flame Blade' spell this is a formidable variant that utilizes a dark fire born of the shadowfell. This spell is best used when direct combat is unavoidable, as while this is a potent spell, it is as reliant on the caster's skill with a weapon as their skill with magic. I recommend that only the brave who have confidence in their sword arm or the desperate who have limited options use this spell. On a side note, be careful not to burn yourself with this spell, though these flames are illusory in nature the necrotic burns they inflict are rather difficult to treat and I write this after having such an experience from testing this spell. I would not like to meet the individual who created this spell.