Break the Immortal

As you say the following incantation, the sky darkens with black clouds.
Your Arcane Focus is fading in dark purple colours.
Your arms are covered in dark marks resembling lightning scars and the air rumbles:
"I call the darkness unto me, from deepest depths of Earth and Sea.
From ancient evils un-awoken, break the one that can't be broken.
To darkest night, I pledge my soul and crush my heart to burning coal.
To summon forth a deadly power, to see my hated foe devoured !"

From your arcane focus, you blast a linear 32 ft/10 m ray of dark energy.
6d6 necrotic damage. They are repelled back 6 ft/2 m.

The spell is predatory to those with a regenerative healing factor.
It blocks it out, preventing them permanently from recuperating.