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The spell Delayed Blast Fireball states:

A beam of yellow light flashes from your pointing finger, then condenses to linger at a chosen point within range as a glowing bead for the duration. When the spell ends, either because your concentration is broken or because you decide to end it, the bead blossoms with a low roar into an explosion of flame that spreads around corners. Each creature in a 20-foot-radius sphere centered on that point must make a Dexterity saving throw. A creature takes fire damage equal to the total accumulated damage on a failed save, or half as much damage on a successful one.

The spell's base damage is 12d6. If at the end of your turn the bead has not yet detonated, the damage increases by 1d6.

According to the concentration rules of The Rules of Magic (emphasis mine):

You can end concentration at any time (no action required).

Is it possible for a caster to cast Delayed Blast Fireball on their turn, end their turn (increasing the damage by a d6 by doing so), and immediately end the spell during the start of the next creature in the initiative order's turn?

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  • \$\begingroup\$ I kinda feel like a good follow up question is: "Why would anyone detonate on the same turn they cast, rather than immediately after their turn ends?" Because the duplicate makes it pretty clear you could cease concentration immediately after your turn ends, before anyone else goes, so the only real benefit to casting immediately would be freeing you to cast another concentration-based spell in the same turn (and there's not a lot of options for doing that anyway, since you just burned your action). \$\endgroup\$ – ShadowRanger Oct 29 at 18:57