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The errata to the DMG added a section "Combining Game Effects" which states:

[...] 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 [...]

There are two similar effects, one is the Stone Golem's Slow feature which states:

The golem targets one or more creatures it can see within 10 feet of it. Each target must make a DC 17 Wisdom saving throw against this magic. On a failed save, a target can't use reactions, its speed is halved, and it can't make more than one attack on its turn. In addition, the target can take either an action or a bonus action on its turn, not both. These effects last for 1 minute. A target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

And the other is the slow spell which states:

You alter time around up to six creatures of your choice in a 40-foot cube within range. Each target must succeed on a Wisdom saving throw or be affected by this spell for the duration.

An affected target's speed is halved, it takes a −2 penalty to AC and Dexterity saving throws, and it can't use reactions. On its turn, it can use either an action or a bonus action, not both. Regardless of the creature's abilities or magic items, it can't make more than one melee or ranged attack during its turn.

If the creature attempts to cast a spell with a casting time of 1 action, roll a d20. On an 11 or higher, the spell doesn't take effect until the creature's next turn, and the creature must use its action on that turn to complete the spell. If it can't, the spell is wasted.

A creature affected by this spell makes another Wisdom saving throw at the end of each of its turns. On a successful save, the effect ends for it.

There is a question on "Can you dispel the Slow effect of a Stone Golem?" where the answer is that you cannot as the Golem's feature is not actually a spell (the features are different features). That said, they do have the same name, so I am unsure how they interact. Many of the effects stacking would be redundant, but your speed could be halved twice and the two effects could have different save DC's.

Do the Stone Golem's Slow feature and the slow spell combine, or does the "Combining Game Effects" section prevent that because they technically share the same name?

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Only "the most potent one" applies

The current rule on D&D beyond is:

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, or the most recent effect applies if the castings are equally potent and their durations overlap.

This is unclear because "The effects ... don't combine" and "the most potent effect" seem to be pulling in different directions.

However, the DMG errata is clear:

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.

Only the effects of one of them [the game features] apply. The DM has to consider the overall effects of each "game feature" and decide which is the most potent - this is entirely a DM judgment call and each DM has to decide for themselves whether one is always "more potent" or if potency is situational. Of course, they can always wimp out and declare them equally potent in which case most recent wins.

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The water's a bit muddy here

The errata (Chapter 8, "Combining Game Effects") you cited regarding the same-name rule goes on to explain what counts as a "feature".

Game features include spells, class features, feats, racial traits, monster abilities, and magic items.

As such the most potent feature would trump the other. Until either feature's effects run out of time, then the longer-lasting one is "reapplied", as per PHB (Page 205, "Combining Magical Effects"):

[...] lnstead, the most potent effect -such as the highest bonus-from those castings applies while their durations overlap.

The DMG-Errata shares the ruling of overlap. However, there is is some conflicting information between the PHB and the Errata:

Where the PHB says:

[...] the most potent effect -such as the highest bonus- [...]

The Errata states:

[...]only the effects of one of them—the most potent one—apply [...]

Given what you rule by, RAW without or RAW with Errata your criteria for what is done actually morphs drastically

What constitutes potency?

RAW by Errata, quantifying a feature's wholesale potency and nullify any misaligmed effects either version may not share with its name-twin is hard or in some cases perhaps even impossible if the features somehow have vastly different effects, but end up being equally "strong". How exactly you rule that is left wide open.

You have to house-rule that. Below you can find how I approach overlapping features.

RAW the DMG-Errata provides no criteria for what makes a feature more potent; however it does point you to what the PHB has to say about that.

RAW the PHB hints at how you can quantify the potency of a spell specifically, it does not use the word feature. If you go pure RAW this is where the trail ends. Reading it Rules As Intended I'd say a spell is a feature and no effect I know of is forcibly required to originate from a spell; so if you can quantify a spell by effect that should work for any feature.

Backed by the PHB's mention of effect-based potency, I compare each individual effect two features of the same name have and then pick the strongest for each. That also constitutes that the unique parts of one feature are compared against "nothing", meaning by virtue of existing they'd trump the lack of any counterpart in their twin.

Using your example:

The golem's ability is a monster ability "feature", the spell is a spell "feature". Since they both fit into that group and share the same name effects that "overlap" would be trumped by the most potent version. The target would also suffer the unique effects from the slow spell feature, i.e. the AC penalty, Dex disadvantage, etc.

And what about the save?

Now, thinking a bit further with regards to DC(s) the PHB's rule of using only the most potent version the DC of the stronger feature user would apply if we go by the definition of comparing each effect individually. So, the save from the amalgamated slow features would be 17 if the spell feature slow is cast by a creature with a spell-save below 17, and the other way around if the caster has a higher spell-save.

Saving whilst both (or more) features are in effect would then end all ongoing effects from both (or all) features of the same name. If one feature ends sooner than the other(s) the highest ongoing save takes hold until there is only one or no more feature.

Shouldn't I have to save from both (or all) features individually?

If you count the saving throw as an effect, no. If you see it as something else, maybe? In a sense both slow features above offer the "effect" of the target being able to perform a save, the only thing that may differ is the number they have to hit.


What if I have one feature with variable effects? For example, would Blindness/Deafness cast twice blind and deafen a creature?

Blindness/Deafness lets the caster choose to inflict one of two effects, each targeting a sense of a creature.

If you follow the DMG-Errata's 'all or nothing' mindset for features the most potent feature-use of Blindness/Deafness would take hold, and if the "lesser" feature-use lasts longer it takes hold once the more potent feature runs out. Already that sounds and feels weird. Let's play out a quick scenario under the assumption that blindness trumps deafness.

Also assume that the target has terrible rolls has to endure the feature-uses to their fullest.

  • The first cast successfully blinds a creature
  • Half a minute into the fight you again cast successfully to deafen it, which does nothing, keeping the blindness as a superior
  • Another thirty seconds later the first cast to blind runs out of time, creature's eyes clear up, it laughs hysterically then realises it's deaf for thirty seconds

Now, that scenario 'works' mechanically, but again; how do you define what feature-use is more potent? Looking for any hints, RAW, there is nothing that would put hearing above seeing or vice-versa. This, again, calls for further house-ruling on these questions:

Is either sense objectively "superior"?

I'd say no, each race / creature has different foci for its senses (bats mostly rely on hearing, giraffes not so much).

Does a given spell (or [perhaps nonmagical] feature, in general) somehow "know" what would affect a given creature most?

Using Blindness/Deafness as an example again, the caster chooses the sense, the feature doesn't state anything like "on a fail the creature loses its most potent sense".

Using the method of 'comparing effects' individually I would follow this train of thought:

  1. Blindness and deafness target a creature's senses, they have that in common

  2. A creature's vision and hearing are specified as different targets

  3. Each effect works on a different aspect of the target creature

  4. Blindness trumps nothing, Deafness trumps nothing

  5. If both saves fail, both effects take hold.

And the DC?

Same as above. Same name, same bane.

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  • \$\begingroup\$ So if you're saying you can do an effect-by-effect comparison wouldn't that mean multiple castings of blindness/deafness would make a character blind and deaf? Oh nevermind, there's the more specific rule "The effects of the same spell cast multiple times don't combine." \$\endgroup\$ – Medix2 Sep 26 at 9:59
  • \$\begingroup\$ Another specific edge case, nice~ I added a bit on that to my answer. That should help, I hope. \$\endgroup\$ – Scrawnoisis Sep 26 at 11:04
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    \$\begingroup\$ You cut off the final sentence of the DMG: "See the related rule in the “Combining Magical Effects” section of chapter 10 in the Player’s Handbook". I mention that fact under "What constitutes potency?" and go from RAW to rules as intended. Mentioning the interactions of spells in a point that focuses on features feels a bit to specific / ignorant of the other types of features. Under the same heading I make the case for ruling features by the same metrics as spells in the PHB. \$\endgroup\$ – Scrawnoisis Sep 26 at 12:05
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    \$\begingroup\$ Ah I see, I must've missed that line, apologies. All good now, thank you \$\endgroup\$ – Medix2 Sep 26 at 14:47
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    \$\begingroup\$ You may want to provide your blindness/deafness argument to this question as well: "If you cast Blindness/Deafness on the same creature twice, what conditions are applied?" \$\endgroup\$ – Medix2 Oct 11 at 15:17

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