Tony Vargas
Legend
Back in classic D&D, when spells could be interrupted when cast, there were also spells that required ongoing concentration that could be broken - sometimes with disastrous results, like your 16HD conjured elemental turning on you. So, no nothing I suggested, above, seems all that bad compared to what it was like back in the day.Certainly, but my concern is that when combined with the concentration rule and "save ends," it creates a situation of double-jeopardy for spellcasters, with multiple opportunities to disrupt their actions and negate the spell. Interrupting a spell was a lot more important when you were stuck with it for a predetermined duration once it was cast.

Six seconds is not a terribly short time compared to how quickly an arrow can be loosed or a sword swung. In six seconds, a high level fighter can attack 10 times (4, doubled for action surge, plus a bonus action attack and a reaction OA. It's not that the fighter is turning into the Flash, at 1st level, he was swinging as fast and as often, it's just that as he levels more of those swings have a real chance of connecting. And, many of those swings are 'interrupted' - parried - as a matter of course all bundled up in the abstraction of the round. That's an idea that's been with the game since the beginning.Also, a round is currently six seconds. Trying to interrupt a spell that can be cast that quickly seems like it'd be just as difficult to interrupt as a regular melee attack.
Spells have never been anything but individual spells, they've never been characterized as many split-second spells, perhaps one of which goes off per round.
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