Recently played with a group, where 3 of their clerics would cast spirit guardians, claiming monsters in the area had to make 3 wisdom saves for 3d8 damage each, potentially adding up to 9d8 damage per round.
Is this any different then trying to have two casters cast insect plague or cloudkill on the same area, or bless on the same individual?
Multiple of the same spell never stack. Player's Handbook, “Combining Magical Effects”:
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap.
With three casters of spirit guardians you can cover more ground, but you will never get more damage or saves out of them by overlapping them.
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap.
If damage is an effect of the spell, funny things can happen. Imagine two clerics (C1 radiant, and C2 necrotic) using this spell, in partially overlapping areas. Two enemies (E1 and E2) are running past them. E2 gets the damage twice, but E1 only once, as the radiant field "protects" him when he would enter the necrotic field.
I really hope this is not intentional.
It seems it is up to us to decide what is an effect and what is not. I think if damage is not an effect, but a separate property of a spell, the game provides better simulation, while still keeping balance.
Two Dire Wolfs bite you, you get the damage twice (2x 2d6+3), but once you are prone, you won't get proner.
Analogously, you are in the area of two Spirit Guardians. You get the damage twice, but your speed is only halved once. Makes sense.
Assume C1 has the Light domain, and he casts Wall of Fire in a 4th level slot, while C2 casts Spirit Guardians in a 3rd level slot. The enemy now takes 3d8+5d8 damage. Casting Spirit Guardians in this 4th level slot would decrease this damage to 3d8+4d8, so it is far from overpowered.
On the other hand, adding two (or more) Blesses together would be clearly overpowered, trivializing GWM for example. But Bless does not do damage, rather it is more like a condition.
You might call it interpretation, you might call it houserule, the bottom line is the game makes more sense if damage is added together, but the conditions (effects) are not.
The simple answer is No, game features do not stack.
As pointed out in @SevenSidedDie's answer, in the Players Hand Book (PHB) the rules for spells are as such:
The effects of the same spell cast multiple times don't combine (PHB Chapter 9, "Combining Magical Effects")
However as some may point out, this seems fairly narrow of a description for what does not stack (e.g. spell effects cast, etc). There may be some room to argue things like two flaming spheres hitting a target at the same time should do double the damage (or in this case, two castings of Guardian Spirits). However, there was an update to the DMG. According to the official Errata:
Chapter 8
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.”
This seems fairly clear that the intended purpose is that no similar spell/ability/effect should occur on a target more than once.
Here is a link to the source material: http://media.wizards.com/2015/downloads/dnd/DMG_Errata.pdf
