How to resolve multiple 'start of turn' triggers

Question

There exist multiple spells/effects/mechanics that can trigger on the start of a character's turn. How in general should a DM rule the ordering, in cases where the order will matter?

As an example: Alice has been downed. She has 0 hit points and has been making death saving throws for a few turns now. She currently has 2 successes and 2 failures. She is also in the area of effect of a Cloudkill spell, which crucially has the following effect:

When a creature enters the spell's area for the first time on a turn or starts its turn there, that creature must make a Constitution saving throw. The creature takes 5d8 poison damage on a failed save, or half as much damage on a successful one.

At the start of Alice's next turn, one of two things could happen, either:

The death saving throw goes first, in which case she has a straight 50/50 of surviving this situation (for now!) or dying right then and there. If she is lucky and gets her third success, the cloudkill is still a concern, but not a matter of life and death. Worst case she is put back into an 'unstable' state but her death saves are reset, buying her precious more time to be saved or evacuated by her party.

or:

The cloudkill check goes first, she dies instantly. Since no matter how well she does on the save to resist it, at best she still takes half damage, and any amount of damage is enough to permanently kill her whilst sitting on two failed death saves.

So whilst in many cases the order of 'start of turn' effects might not matter, in at least this example we have a case where the permanent death of a character could be drastically effected by the order ruled by the DM.

So how should a DM rule the ordering of such effects?

Answer 1

(In this case) The Player controls the ordering

Xanathar's Guide to Everything includes an (optional) rule for exactly this situation. On page 77, it contains a section entitled "Simultaneous Effects":

In rare cases, effects can happen at the same time, especially at the start or end of a creature's turn. If two or more things happen at the same time on a character or monster's turn, the person at the game table - whether player or DM - who controls that creature decides the order in which those things happen.

So in this case, Alice's player would control the ordering. Odds are good, he or she would select the Death Saving Throw to come first.

Answer 2

As the other answer mentioned, Xanathar has rules for this. However, if you do not want to enforce the optional rule for some reason, our table has used the following solution in the past:

By the order the events happened.

So, in your specific case, if Alice was knocked unconscious before someone cast the spell, she does the Death Saves first. Otherwise, she takes damage first.

It's a simple solution, and it feels elegant to our table (only came up once and that was how it was resolved).

Answer 3

Initiative order can give guidance

It is important to consider that these effects are simultaneous. The 6 second round encompasses all actions during that period and they are effectively simultaneous. The Initiative order helps to organize that time in a more approachable way.

But in this case, there aren't any specific rules on how to adjudicate simultaneous events. This leads it to being either a DM decision or looking at other rules for guidance. Because Initiative is the means for breaking down simultaneous events, it becomes the closest guiding principle.

(PHB, 189)

Initiative determines the order of turns during combat... This is the order (called the initiative order) in which they act during each round. The initiative order remains the same from round to round.

If a DM doesn't want to make the judgement call on Life or Death in this situation, they can refer to the general guidance of order of actions and let those actions play out in the initiative order.

DM decision

However, applying general initiative rules to such a case may not seem reasonable to some DMs. Giving the player a chance to save their character before automatic death due to damage is not unreasonable.

Answer 4

I don't believe there would be a hard-and-fast rule for this as there are just too many permutations to consider, so I would go in order of specific rules to general situations.

Specifically, Alice drowned and therefore must make three death saves. This would come first. In your scenario, she has already done this for 4 previous rounds. If she fails the final check, then she would not be breathing and therefore is not likely to breathe in the poison of the Cloudkill spell.

As a general condition, Alice is in a Cloudkill spell and will be subject to it's effects. So, if she makes the final death throw save, the first thing she will do is take a deep breath of poison; killing her all over again.

It should also be noted, under "Instant Death"

Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.

IF Alice makes the final death saving through, she will still be at 0 hit points and unconscious. Cloudkill can do up to 40 hp of damage (baring any other bonuses). If Alice is a low-level mage with only a meager hit point maximum her struggle to survive would be for naught.