The Earthquake spell creates an area of intense tremors. Creatures within the area are knocked prone:
When you cast this spell and at the end of each turn you spend concentrating on it, each creature on the ground in the area must make a Dexterity saving throw. On a failed save, the creature is knocked prone.
The creatures are knocked prone "at the end of each turn you spend concentrating" on the spell. Interestingly, it doesn't state that these turns have to be your own. Presumably, you keep concentrating through everyone's turn, and so the Earthquake spell can knock prone at the end of every turn, including those of your allies and enemies. Is that correct?
Best Answer
It says turn, but it probably means your turn.
If you want to apply this spell from a strict and literal reading of the rules, then sure, every creature makes the saving throw every turn.
However, this creates a highly unusual phenomenon: the power of the spell scales with the number of combat participants. Every creature that rolls initiative in a combat would increase the number of saves per round made by a victim of the spell. This makes exactly zero narrative sense. Given this, it seems clear enough that the spell should say something like “at the end of each of your turns”, so that the number of saves a creature makes per round is independent of the number of participants in combat.
So the way I would rewrite the spell description, and the way I have always run this spell, is:
Now, there is a rule we can draw support from to motivate this ruling - a combat round is always six seconds:
The idea here is that a round is always six seconds no matter the number of participants. Because of this, we can see that narratively, each individual turn in a round actually is happening at essentially the same time. In the fiction of the game, each creature’s turn is not a discrete event at a different time, all the turns are occurring at the same time, with initiative representing how quick each participant is to act in each combat round.
Thus, we should expect each casting of earthquake to produce a similar effect requiring the same number of saving throws made every six seconds that the spell persists. The idea that earthquake produces an arbitrarily large magnitude of effect scaling with the number of creatures that happen to be participating in combat is simply inconsistent with the narrative the combat rules are trying to represent.