I have researched the matter and found only this question (which seemed relevant but does not answer the exact question we're having): Quickened spells and concentration
Witch Bolt (PHB, p. 289) is a 1st-level sorcerer spell with a casting time of 1 action that requires concentration. The spell also states:
On a hit, the target takes 1d12 lightning damage, and on each of your turns for the duration, you can use your action to deal 1d12 lightning damage to the target automatically.
The Quickened Spell metamagic option for Sorcerers states simply:
When you cast a spell that has a casting time of 1 action, you can spend 2 sorcery points to change the casting time to 1 bonus action for this casting.
One of my players argue that Quickening the spell would allow them to use a bonus action to instantly attack the target on each subsequent turn.
Is there any official ruling on this anywhere? Is the difference between casting a spell and using the spell on subsequent turns while concentrating clearly explained anywhere?
I have no doubt that "casting the spell" and "using it while concentrating on it" operate differently and it would have to be specified in the metamagic feature if it had an impact on the concentration aspect of the spell, but I'm looking for an official written rule.
Thanks in advance.
Best Answer
Quickened Spell only affects the initial casting of the spell; it has no effect on actions granted by the spell
The Quickened Spell Metamagic, as you quoted, does exactly what it says: it changes the casting time of the spell from an Action to a Bonus Action. It does not do anything else.
The effect of Witch Bolt, which includes the ability to continue to deal damage by simply using the caster's Action, is not part of the spell's casting time. It doesn't recast the spell, as some people misinterpret the spell to behave. It's just an Action granted by the ongoing effect of the spell. Therefore, the spell still requires the spellcaster to use their Action, not their Bonus Action, to trigger the damage, even when quickened.
This does introduce a curious interaction between Witch Bolt, which says that the spell ends if the spellcaster's action is used for something else, and when cast as a quickened spell, the spellcaster's action might not have been used. So if the caster then immediately uses their Action to do anything other than trigger bonus damage against the target, the spell would end.
But that's just a property of the spell's requirements to stay active. It's still an Action, and Quickening the spell does not change that.