a) If the creature is in the area when you cast the spell. b) At the end of its turn, but only if it remains in the area. If the creature moves out of the area during its turn, it won't need to make the save. c) Also if the area enters a creature's space after casting the spell.
In any case, "a creature makes this save only once per turn."
I think this version was the attempt at a compromise where you can cast a spell and at least on one turn of guaranteed damage (the turn where you create the spell), but gives the opponent agency to move out of the area on its turns to prevent future damage. The caster can move to get more damage out of the spell, but the opponent always has the opportunity to move also to avoid some of the damage.
In general, except for grapple shenanigans, it feels fine in comparison to 2014, where every spell had its own rules and you had to remember which was which and sometimes you'd cast a spell and get nothing out of it because they'd move before they took damage or only ever take damage once or whatever.
If an enemy is next to you when you cast spirit guardians is it hit twice in the first round if it does move out of range.
whenever the Emanation enters a creature’s space
and whenever a creature enters the Emanation
or ends its turn there,
To look at it another way, when you initially cast it, does it spell enter everyones space, and so you roll damage when you cast it?
The creature must make a Wisdom saving throw:
a) If the creature is in the area when you cast the spell.
b) At the end of its turn, but only if it remains in the area. If the creature moves out of the area during its turn, it won't need to make the save.
c) Also if the area enters a creature's space after casting the spell.
In any case, "a creature makes this save only once per turn."
So (2024 rules) you are saying yes it can hit twice on the first round
- first time on the clerics turn
- again on its turn if it sticks around
Its just a lot of damage.
It depends, really. This thread compares several scenarios between the 2014 and 2024 behavior of Moonbeam: Question about the new rules of when creatures take damage from ongoing spell effects - Rules & Game Mechanics
Moonbeam behaves similarly to spells like Spirit Guardians, Conjure Animals or Cloudkill as well as others with similar wording.
I think this version was the attempt at a compromise where you can cast a spell and at least on one turn of guaranteed damage (the turn where you create the spell), but gives the opponent agency to move out of the area on its turns to prevent future damage. The caster can move to get more damage out of the spell, but the opponent always has the opportunity to move also to avoid some of the damage.
In general, except for grapple shenanigans, it feels fine in comparison to 2014, where every spell had its own rules and you had to remember which was which and sometimes you'd cast a spell and get nothing out of it because they'd move before they took damage or only ever take damage once or whatever.
@Greg07151 besides the link I posted earlier, there are are some interesting recent threads about your thoughts or concerns: