I was running an adventure last night that had Ice Trolls.  Each one has a cold aura that does 10 damage.  Four of them ended up swarming the party.  At first I played that they stacked, but quickly realize that this was going to most likely end in a TPK.  So, that made me change my mind that they should not stack.  I tried to do a quick search during the game but I had a hard time finding an official ruling on how they should work.

Can someone point to me how monster auras work?  Do they stack?

Thanks,

Madgertick

An aura is a passive feature that is always in effect (unless the feature specifies otherwise). Different overlapping auras can stack, but the same effect overlapping from multiple sources does not. I'll try to find the rule or SAC entry that says so specifically.

EDIT: Here it is. It was in an errata, so I couldn't find it in my old physical DMG.

EDIT2: Thanks David42! And here's the actual errata document from the WotC website freely available to all with the text of the rule change.

Here is the text of the errata in case you don't have the DMG on D&D Beyond.

"Combining Game Effects (p. 252). This is a new subsection at the end of the “Combat” section:
Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap. For example, if a target is ignited by a fire elemental’s Fire Form trait, the ongoing fire damage doesn’t increase if the burning target is subjected to that trait again. Game features include spells, class features, feats, racial traits, monster abilities, and magic items. See the related rule in the “Combining Magical Effects” section of chapter 10 in the Player’s Handbook"

So in the case of the Ice Troll, characters would only be affected by one of them on their turn.

Short answer, if the ice troll Cold Aura stated that it triggered at the beginning of the Ice Troll’s turn then they would stack because each ice troll has their own turn. Since it states that it triggers at the beginning of the other creatures’ turns they don’t stack because they all trigger at once.

Here's the text for an Ice Troll's cold aura:

"Cold Aura. While it's alive, the troll generates an aura of bitter cold that fills the area within 10 feet of it. At the start of the troll's turn, all nonmagical flames in the aura are extinguished. Any creature that starts its turn within 10 feet of the troll takes 10 (3d6) cold damage."

By this wording and the general rule about multiple copies of the same effect, if a creature starts their turn within the aura of multiple Ice Trolls, it would still only take one instance of the damage. If you want to think of it in in-game terms, multiple Ice Trolls can overlap their Cold Auras to make one larger Cold Aura, but overlapping them doesn't make that part of the aura any colder, so the amount of damage taken doesn't change. 

Thanks, all!