[RPG] If you’re concentrating on one spell, and you cast another concentration spell, when exactly does the first spell end

concentrationdnd-5edurationspellcastingspells

This came up last night in our D&D session.

Situation: A wizard cast Evard's Black Tentacles; another creature fails its Dexterity save and is restrained by the tentacles. This spell requires concentration.

While maintaining concentration, the wizard casts Wall of Fire on a good chunk of enemies. This spell also requires concentration.

Are the enemies still "restrained" by Evard's Black Tentacles when the Wall of Fire is initially cast, and do they have disadvantage on Dexterity saving throws against the Wall of Fire? Or does casting the spell immediately end the Evard's Black Tentacles spell, giving the creatures enough time that they are not restrained and thus they do not roll the Dexterity save at disadvantage?

Both of these spells require concentration so they cannot both be active at the same time.

Best Answer

A spell ends when you lose concentration:

Some spells require you to maintain concentration in order to keep their magic active. If you lose concentration, such a spell ends. Phb 203, emphasis mine.

So if you lose concentration the spell ends, no conditional time given. And we can also see what breaks concentration on the same page:

You lose concentration on a spell if you cast another spell that requires concentration. You can’t concentrate on two spells at once. Ibidem, emphasis mine.

My interpretation of this is that both spell effects can't be active at the same time, thus the enemies won't be restrained.

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