Witch Bolt 5e Spell Description
When A beam of crackling jump into the blue, energy lances toward a creature within the range, then automatically forming a sustained arc of lightning between the target and you. Now you can set the target and make a ranged within spell attack against that creature.
On One hit, the target will take the 1d12 lightning damage, and now you may use the action to deal with 1d12 lightning damage to the automatic target on each of your turns for the duration of time.
If you want to do use your activity for something else, then the spell also ends. The spell can even get finished if your target is outside your limit or if you have it for total cover from you.
At highest Level : When you are going to cast this spell for the above slot of 2nd level or higher level, the initial damage gets increased by the 1d12 for each slot of the level above 1st.