Vile Lightning

Make a melee spell attack against a creature you can reach. On a hit, the target takes 2d8 lightning damage. Each creature within 5ft of the target, including the target must make a CON save or take 1d8 necrotic damage. If any of the additional creatures fail, the vile lightning can chain to an additional creature within 5ft of it. Have those creatures make a CON save, taking the 1d8 if they fail and chaining again. 

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the necrotic damage increases by 1d8 for each slot level above 1st, this additional damage die applies to each target the spell chains to.