Ok, so I admit I had my own homebrew version of Witch Bolt (which I actually broke up into 2 spells, Witch Bolt and Lightning Lasso). But after reading some of the comments here, I think I was inspired to write up a new version that I think is even better than what I previously came up with.
Witch Bolt
1st-level evocation
Casting Time: 1 action
Range: 30 feet
Components: V, S, M (some iron filings)
Duration: Concentration, up to 1 minute
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. You have advantage if the target is wearing armor made of metal. On a hit, the target takes 1d12 lightning damage. As long as you maintain concentration on the spell, on subsequent rounds you can use your action to deal 1d12 lightning damage to the target automatically, even if the target gains total cover from you after the initial damage.
For the duration of the spell, the target must succeed on a Strength saving throw in order to move beyond the range of the spell. On a success, the target can move normally. On a failure the target can still move normally, but until the start of their next turn they cannot attempt to willingly move beyond the range of the spell. This spell does not prevent the target from teleporting beyond the spell’s range, nor does it prevent forced movement such as being the target of the
thunderwave spell.
The spell ends if the caster and target are ever farther from each other than the spell’s range.
At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the damage increases by 1d12 for each slot level above 1st.
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So what changed?
1) The initial attack has advantage against creatures wearing metal armor. If it works for shocking grasp, then why not witch bolt?
2) The spell doesn't end if you use subsequent actions for other things, or if the target gains total cover after the initial casting. This is because I think maintaining concentration is enough of a tax to maintain the spell, and I like the idea of the spell continuing around corners or behind obstacles.
3) The target can't willingly move beyond 30ft without succeeding a Strength save. I think this is less complicated than and less powerful than grappling the target. It also makes me think of wizards engaged in spell duels while also being tied together. And meanwhile, the typical fighter will just walk away or decide to punch the caster in the face. Also good for locking down flying enemies that get too close.
4) Upcasting increases initial damage AND ongoing damage.
So cast at 1st level, this version offers some interesting battlefield control options, thus justifying the low damage. At higher levels, the damage potentially really gets up there, but given that the caster is basically tying themselves to whatever they have targeted, it puts an even bigger target on their back.
This witch bolt is basically the beam from a proton pack.
Update 1: Added that the Strength save can only be attempted once per round. Also in the wording it's implied (I hope clearly) that while the target can't move beyond the range of the spell, it doesn't prevent forced movement (such as being dragged by someone or blown back by something like thunderwave).
Update 2: Just a general re-wording. Hoping it is more clear while maintaining natural language as much as possible.