I'm not sure why you'd start in your quixotic quest to claim that concentration is not an overused mechanic by focusing only on the damage & damage+effect spells mentioned with such a weak argument & ignoring everything else.
It's a second level spell castable at 4th level for 2d6 fire...
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Yes you can upcast it to 4d6 using a 4th level spell slot at level 8 or higher to get that to the "4d6" fire number you cite
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are you seriously claiming that this is an "win button" when rogue is doing more than that every round or that any of these are a "win button".
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Sure It's "useful" damage just like the 2d6 & 4d6 of all those other classes& nobody is claiming otherwise, but not so over the top that it needs a short duration
and a spell slot
and concentration
and a regular bonus action... It's flaming (2d6 fire) sphere, not disintegrating sphere or burning rooting sphere. There are multiple threads on revising fighter who is one of those examples, you might want to take your
"If you can’t make that work, then that’s on you " over to those & point out how they can use that "win button" every round of every fight goes over in those now that you've tested it out over here.
Since you bring up the Bigbys referenced by someone else too.. Yes it also can do 2d6+casting mod plus grapple as one of the things it can do(grasping hand option) each round for the short 1 minute duration
and use a bonus action
and 5th level spell slot
and maintain concentration. Hardly the "win button" condition you make it out to be. given how all those other classes up there can do the same every round of every fight without a spell slot usage. Going back to all of the other spells crippled or made useless by the excessive & needless application of concentration, once concentration is applied the balancing needs to shift from "is this effect balanced against the action, slot cost. & duration" to "Is this effect & action cost balanced against the fact that the caster has so much of their arsenal blocked off when using it given the slot cost, action cost, & durations the spell allows" & wotc did not do that when they took X over time spells that were balanced entirely around duration & slot cost then added concentration while shifting move action costs consuming something the caster generally had no need for otherwise to bonus action costs the caster very much could be using every round for something else just like the other classes with their bonus action. Wotc's method maps fine using very specific types of spells such as direct damage/instant nukes where the only balancing factors are "does it do enough damage to one target or an area of targets when cast to justify the spell slot", there's no question of additional over timeor nondamage effects & it's very easy to "balance". A Damage over Time (DoT) is a little easier to balance, but when you allow an unrepentant
munchkin design things like
this & design concentration to wall off anything else a caster could be doing so that a 20d8 no save second level spell is only rising to the level of being kinda broken without cooler heads saying "well no bob that really hurts casters who aren't just blasters & you can't do that even if it makes a blaster feel really awesome".