Durren's Pulse Burst

Concentrating on a point the static is drawn from the atmosphere, and a pulse of lightning apparates, marking the creature with a wire of lightning. The target must make a Constitution saving throw. A creature takes 4d6 lightning damage on a failed save, or half as much damage on a successful one. 

Any creature that fails the saving throw has its speed reduced to half and will be dealt an additional 2d6 of damage at the start of their turn until the spell ends. At the end of each of their turns, the target may make an additional Constitution saving throw to escape the effects of the spell. The spell ends in 1 minute, if concentration is broken, or if the target escapes.

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the damage increases by 1d6 for each slot level above 2nd.