As written, witch bolt is a nicely thematic, flavorful spell which is nice enough for low-level characters, but falls flat after only a few character levels simply because the ONLY damage in it that scales up is the initial d12... Everything after that is apparently just a mere 1d12, no matter what level slot you cast it with.
Was this intentional? If so, it's a bit of a shame, because otherwise it's a great little spell with a lot of potential.
(First I thought it would be very strong and unbalanced, but after the comment of GreySage, I started calculating, and the numbers did not support my assumption)
While I am not aware of any statement from the creators, it is highly unlikely that the follow-up damage was meant to scale, as most 1st and 2nd level spells scale horribly.
(I will call the proposed change WB+)
I compare WB+ to Phantasmal Killer and Sunbeam, calculating for the first 3 rounds of combat, as encounters usually don't last longer. Concentration is assumed to hold.
WB+ positives:
Negatives:
Damage comparison
At level 7, when you get 4th level slots, DC 15 is common (8 + 3 proficiency + 4 ability). The expected duration of Phantasmal Killer is 3 rounds against a Goblin (Wis -1) but only 1 round against a Stone Giant (Wis +4). I will calculate with 2 rounds.
The expected monster AC is 15, the spell attack bonus for the same wizard is +7, so the hit chance is 65%.
The full damage with Phantasmal Killer during the 3 rounds is 58.3:
WB+ is easy, 3 * 0.65 * 4d12 = 50.7.
WB+ positives:
Negatives:
Damage comparison
At level 11 the DC is 17, I will calculate with 30% Con save chance. So in three rounds against one enemy it is 68.85 = 3 * (6d8 * 0.7 + 27 / 2 * 0.3)
WB+ has the same hit chance as before, the spell attack bonus increases to 9, but the monster AC also grows to 17. It is 3 * 0.65 * 6d12 = 76.05
WB+ looks good at first glance, but if you can get a second target into the area of Sunbeam, it wins easily on damage alone. Add to this the blinding and the ability to use Sunbeam even after the original target is dead.
At higher levels it is much easier to restrain enemies, preventing them to leave the range of Witch Bolt.
At the same time, Magic Resistance and Legendary Saves are more frequent.
It would not be game-breaking to allow the secondary damage to scale. The DMG' definition of overpowered:
If a spell is so good that a caster would want to use it all the time, it might be too powerful for its level
I would definitely not want to use it all the time, even if I had an optimized grappler in my team, and most casters don't.
I have played in a game where the DM allowed our Warlock to scale the continuing damage as well. I can say that the spell was very good, but it did not reach what I would call over-powered. It manages to outscale pretty much any other level 1 or 2 spell, but due to taking an action each turn does not really outscale later level spells in their own slots.
Overall I would answer: it was intentional to prevent the spell from outscaling Magic Missile, Chromatic Orb, Ice Knife, etc. I would agree that that's kind of a shame; the spell does not break the game if allowed to fully scale.
I doubt it is an omission. It increases by 1d12 per level, which isn't exactly terrible. Magic missile increases by 1d4+1. If you have a 65% hit chance, 1d12 gives you 4.225 damage per slot. Compare that to magic missile which does an average of 3.5 per slot.
I agree that the "lingering effect" isn't good later on, but most level 1 spells scale pretty poorly.
