Witch Bolt (Greater)

A beam of crackling, blue energy lances out toward a creature within range, forming a sustained arc of lightning between you and the target. Make a ranged spell attack against that creature. On a hit, the target takes 1d12 lightning damage, and on each of your turns for the duration, you can use your action to deal 1d12 lightning damage to the target automatically. The spell ends if you use your action to do anything else. If the target is outside the spell’s range or behind total cover from you when it ends its turn, it can make a Constitution saving throw against your spell save DC, breaking the connection and ending the spell on a success. If the target is outside the spell's range or behind total cover from you when you end your turn, the spell automatically ends.

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the initial damage increases by 1d12 for each slot level above 1st. Each consecutive turn that you use your action to continue the spell, the damage dealt by the witch bolt decreases by 1d12 (to a minimum of 1d12). For example, if you cast the spell at 3rd level, the initial damage is 3d12. On the next turn, it is 2d12. Every turn after that, it is 1d12.

If your witch bolt is still active, you may use your action to expend an additional spell slot up to the level of the original spell slot cast, increasing the damage that the witch bolt deals on that turn to 1d12 per level of the new slot expended, no attack roll required. The damage then decreases by 1d12 per round as usual.