When exactly do "once each round" effects work?

Question

Quite a bit of spell effects state that they work once in a round.

For example, Balor Nimbus (Spell Compendium p24):

each round, the flames deal 6d6 points of fire damage to any creature grappling you (or any creature you grapple) on your turn.

When exactly on my turn does the effect happen? Am I right assuming that the effect happens once on my turn at the earliest possible moment? There ought to be a clear ruling somewhere, I just can't find it.

To clarify a bit, consider this situation (Initiative order is X then Y then Z):

1. On round 1, X casts Balor nimbus (standard action), then Celerity (swift action), then starts a grapple agains Y (standard action from Celerity) and gets a hold. Does his opponent get burned right away? Or does the damage occur at the beginning of the round?
2. On Y's turn he breaks free.
3. On Z's turn Z starts grapple with X and gets a hold.
4. Next round, X breaks free from grapple (standard action). Does Z take damage from Balor Nimbus?

Bonus question, to complicate things more:

1. X has Improved Grab ability and Balor Nimbus on. X grapples Y with one hand.
2. It is start of X's turn. Does Y take damage from Balor Nimbus? If yes, when X grapples Z with his other hand, does Z take damage from Balor Nimbus?

If Balor Nimbus is too hard, let's try Wall of Fire.

TL;DR

"Once each round" effect: happens at the beginning of the round or at the earliest possible moment at the round? Goes off once at some moment in time or once for each creature affected, possibly at different moments?

I'm interested in an answer supported by the rules.

Answer 1

Basically, so far as I can tell, they happen literally n rounds later, on the same turn they started on. If you first took damage from Balor Nimbus because you started grappling it, you’re taking damage on the beginning of every one of your turns. On the other hand, if you’re taking damage because the balor grappled you, you’re taking damage every round on his turn. In all cases, this should be the beginning of the turn in question.

You should not be able to take damage from these effects more than once per turn, so if you end the grapple on your turn, and the balor starts it up again, you just take damage again next round as if you’d never left.

But so far all of this is just based on what the unit of time known as “one round” literally means, not any rules citation. Still looking for that.

Answer 2

Okay, completely misread the question - I thought it was talking about a Balor's special ability, not a spell ( points to you sir, for finding a spell I didn't know off the top of my head! ). Even so, I stick by my answer: At any point in time during the player's turn that he wishes for it to happen. Usually the first thing, but I can think of some instances where delaying it would be beneficial. The spell itself doesn't list when it happens, but we do get some insight from the description:

... to any creature grappling you (or any creature you grapple) on your turn.

Meaning that if any creature is grappling you it makes "when" not matter, because they can't let go during your turn they take damage. If you grapple a creature it, again, makes no difference as to "when", as you could start the round with them grappled and deal the damage or end the round with them grappled and deal damage, as both are still "on your turn" but allow you to grapple someone during a round and deal the damage, or grapple someone the round before ( potentially dealing the same damage ), deal damage to them again, then let them go and grapple someone else.

Honestly, "when" seems irrelevant to me. It's your turn, and you can decide the order of self-generated actions as you please.

Update: 1. Yes, Y gets burned right away. It is X's turn, and he has grappled someone. -- 4. Again, yes Z takes damage. It is X's turn, and he is grappled. Bonus #2: Yes and Yes, they both take damage.

As to Wall of Fire, that's is no different at all, same thing. It only specifies that it happens on your turn, not specifically when - or ever once, for that matter.

Answer 3

The effects take place on the following round during the initiative score on which they were cast or activated. If you cast Balor Nimbus the damage takes place on the initiate score you begin your grapple or when the creature in question grapples you.

So if Jimmy the hat tries to strangle your cleric on Initiative count 27, on the following round when Jimmy would try to strangle you more, he would take more damage.

The text for wall of fire states:

One side of the wall, selected by you, sends forth waves of heat, dealing 2d4 points of fire damage to creatures within 10 feet and 1d4 points of fire damage to those past 10 feet but within 20 feet. The wall deals this damage when it appears and on your turn each round to all creatures in the area. In addition, the wall deals 2d6 points of fire damage +1 point of fire damage per caster level (maximum +20) to any creature passing through it.

So if Bob the Wizard has an initiative of 12 and casts the spell after Jimmy the hat moves to strangle your cleric, The fire damage from the heat of the wall takes effect on initiative 12 burning your cleric and Jimmy. On the following round, if Jimmy the hat gets to move on Initiative count 27, and decides to pass through the wall, he takes an additional 2d6+1/lvl points of damage for passing the wall.

Your effects list would look something like this:
27 Jimmy the Hat ( Balor Nimbus, Grapple damage )
26
...
13
12 Bob the Wizard ( Wall of Fire damage )

If a few rounds later you land a Hold Person on Jimmy the hat and then give him a nice bear hug the effect for Balor Nimbus takes effect when you initiate your grapple instead.