Arm tear

The caster takes up his/her arm and stretches it out pointing at a target with an open hand. Make a ranged spell attack,  on a hit the target takes 6d10 force damage and is pushed 20 ft back with a loud boom that is heard up from 600ft of the source, the target has to make a con saving throw with advantage or lose a limb or head from the table below. (Losing a limb has the effects from lingering injuries in the DMG.)

1d20 

1-9 The target loses a leg 

10-19 The target loses an arm

20 The target loses it´s head and dies unless it can survive without it´s head.

At higer levels: For each spell slot above 5 that you cast this spell for the damage increases by 1d10