Choke of the Dark Veil

The Choke of the Dark Veil is a powerful spell, taught often among the ranks of the dark wizards.

The wizard who uses this powerful attack lunges forward up to 50ft away, and turns into a swirling black mass, in midair. Upon reaching the target, the black mass envelopes the target, destroying the mind and devouring the soul of whoever should fall victim to its leap.

Upon contact with an enemy, deal 20d12 psychic damage upon a failed wisdom saving throw, or half as much on a successful one, and they receive the restrained, petrified, and paralyzed conditions.

After the initial strike, madness engulfs the caster, and they are no longer able to do anything, except continue to destroy their target. Until the caster is released, the target will take 14d12 psychic damage every turn, and they may not move or be moved, heal or be healed, or be buffed until the caster is released. As the swirling black mass and it's victim fuse together, the caster may no longer leave its target, until one of the two eventually dies. 

While the spell is active, the caster may not make any saving throws, being invulnerable to wisdom, charisma, and intelligence, while being a easy target towards strength, dexterity, and constitution. Any damage given to the caster while the effects of the spell are active radiates through the victim twofold, however damage rolls against the caster have advantage. 

As long as the victim dies before the caster, after the spell, the caster's form slumps to the ground as it transforms back, and the corpse of the target does not appear back. 

The caster takes 1d10 psychic damage for every turn it took to defeat the enemy (this cannot kill the caster), gains the exhaustion condition until the next long rest, and can no longer cast spells of level 5 and above for that period of time. The caster is also unconscious for the next five turns if the battle is still going, and cannot be healed by any means until this time is over.

Finally, some minor rules:

No two party members may cast this spell on the same enemy at once

The vial of blood must be the caster's own, not another being's

And lastly, the DM is allowed to alter or ban any of the above features, as this is a fairly overpowered spell, despite the attempts I made to balance it, such as the advantage on all damage checks against you.

Hope that you guys enjoy using this spell, I have worked very hard on making it, and am rather proud of my creation

Happy Dragon-Hunting!