Black flame of the abyss

The Spellcaster stretches their arm out with a fist. Then to open it after arm is fully extended the fist is to open forming a symbol of unknown origin but similar to the style of ancient symbols from the abyss. The spell caster will then grip the blade of the wepon with the palm of the hand transfering the symbol to the wepon. Where it is then absorbed by the wepon then from the position of where the symbol was absorbed a dark translucent energy based shield covers the blade. After the entire blade bursts into a dark flame only exposing the fuller. The spell adds 1d4 of damage and causes a burn effect that causes 1d4 of fire damage every turn. Due to the nature of the spell a d20 is rolled at each cast and If a 1 is rolled the wepon cracks and abyssal texts scrawl across it, from this out the cracks black flame leak out for 1 second. After this the wepon will combust causing damage to anything in a 10foot radius of the wepon (3d6 on failed dexterity save of dc11 no damage on succeeded save ). The second stage of the spell causes the effect of the spell grows causing the Spellcaster to receive 1d8 of psychic damage and making them more mentally unstable as their intelligence and wisdom drop to 6 if stat is above and stays the same if lower or equal to 6, strength and dexterity increase by 10 to a maximum of 22 and if stat is above 22 then there is no change. But the strength of the wepon is increased by an additional 1d6 and flame damage is increased by an additional 1d4. But the Spellcaster receives 1d6 of flame damage . The change in aesthetic is that abyssal texts reading "from the lowest point of infinity this power was drawn from and it shall bring you back as payment" scrawl over the Spellcasters hand and forearm and the entire wepon becomes covered from top to base in the spells effect. After the spell is undone the entire wepon turns into coal and slowly crumbles making it useless. Aswell second stage Lasts 12 rounds and is suddenly turned to coal afterwards. At higher levels:

5th level- added damage increases by a d4 (2d4) and the flame damage increases a d4 (2d4)

11th level-added damage increases by a d4 (3d4) and the flame damage increases a d4 (3d4)

17th level- added damage increases by a d4 (4d4) and the flame damage increases a d4 (4d4)

Second stage:

5th level- added damage increases by a d6 (2d6) and the flame damage increases a d4 (2d4)

11th level-added damage increases by a d6 (3d6) and the flame damage increases a d4 (3d4)

17th level- added damage increases by a d6 (4d6) and the flame damage increases a d4 (4d4)