Dark Rune of Morik

You quickly carve a rune into the palm of your hand. The rune then begins emitting black and red vapour.  Extending your hand to expose the palm, you send forth a blast or vile energy, forming a line of 100 feet long and 5 feet wide in a direction you choose. Each creature in the line must make a dex saving throw. A creature takes 6d8 necrotic damage on a failed saved, or half as much on a success save. 

On each of your turns for the duration, you can use your action to fire off another blast of energy from the rune. Each subsequent time you do so, the necrotic damage increases by an additional 1d8,to a maximum of 9d8. However the pain from the rune begins to take hold. Each time you fire off another blast after the initial one, you take 1d8 necrotic damage. The necrotic damage ignores resistance and immunity, but doesn't force you to make a constitution saving throw to maintain concentration.