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In Volo's Guide to Monsters, the Yuan-Ti "Unusual Abilities" section (p. 98) lists a number of action options that might be given to high-ranking yuan-ti, one of which is:

Polymorph into Snake (Abomination or Malison Only; Recharge 6). The yuan-ti targets a creature it can see within 60 feet of it. The target must succeed on a Wisdom saving throw or be transformed into a Tiny poisonous snake, as if affected by the polymorph spell. The save DC is the same as that of the yuan-ti's Innate Spellcasting ability.

It's not stated that you cast polymorph; it says that the target is under the effect of the spell. So how I should interpret this? Does it require the yuan-ti to maintain concentration? What is its duration?

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Its duration, including whether it requires concentration, is unspecified.

Polymorph Into Snake imitates the effect of Polymorph, not any other aspect of it, and duration is not part of a spell's effect.

Casting a Spell:

Each spell description begins with a block of information, including the spell's name, level, school of magic, casting time, range, components, and duration. The rest of a spell entry describes the spell's effect.

Then, under "Duration":

A spell's duration is the length of time the spell persists. A duration can be expressed in rounds, minutes, hours, or even years. Some spells specify that their effects last until the spells are dispelled or destroyed.

...

Some spells require you to maintain concentration in order to keep their magic active. If you lose concentration, such a spell ends. If a spell must be maintained with concentration, that fact appears in its Duration entry, and the spell specifies how long you can concentrate on it. You can end concentration at any time (no action required).

Therefore it has no specific duration and may or may not require concentration.

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  • \$\begingroup\$ The spell effect refers to the duration, how does this answer solve that problem? "The transformation lasts for the duration, or until the target drops to 0 hit points or dies." \$\endgroup\$ – Akixkisu Jul 6 at 17:17
  • \$\begingroup\$ @Akixkisu If someone has table experience with handling spells that don't specify a duration, I would welcome their input. \$\endgroup\$ – Mark Wells Jul 6 at 17:51
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    \$\begingroup\$ I'm in agreement with the points Akixkisu's has raised. I fail to see how you've managed to ignore the duration with respect to this: as if affected by the polymorph spell. It doesn't say effect, it says affect, and no they aren't synonyms. Affect is a verb, it's an action that occurs. Effect is a noun, it's the description of what's occurring. Since it's affected by the polymorph spell in the description of the ability, it means everything from polymorph applies. If it had the same effect as a polymorph spell that would mean it was simply transformed. So yeah, effect =/= affect.' \$\endgroup\$ – Lino Frank Ciaralli Jul 6 at 18:45
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    \$\begingroup\$ @Rubiksmoose My point is that it isn't actually the Polymorph spell. It affects the target as Polymorph would (that is, has the same effect) but it doesn't have the spell's other characteristics, such as level, range, components, or duration. \$\endgroup\$ – Mark Wells Jul 6 at 19:40
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    \$\begingroup\$ That's the contention though Mark. How did you come to that determination? Because if I cast Dispel on the target, you'd have to roll or upcast it to end the effect since Polymorph is a 4th level spell. \$\endgroup\$ – Lino Frank Ciaralli Jul 6 at 21:01
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Polymorph into Snake has a duration of up to 1 hour based on concentration.

From the VGtM yuan-ti Action Option Traits (98):

The target must succeed on a Wisdom saving throw or be transformed into a Tiny poisonous snake, as if affected by the polymorph spell.

Taking 1 action (it is an action-option), the yuan-ti uses their (unusual) ability Polymorph into Snake. The target of the transformation of Polymorph into Snake is as if affected by the Polymorph spell.

For the target to remain polymorphed the Polymorph spell requires concentration by the caster of the Polymorph spell. In this case, that is the creature that uses of Polymorph into Snake because of the reductio ad absurdum that if the yuan-ti is not treated as the caster, then the effect would immediately end due to the requirement of the caster maintaining concentration for the Polymorph spell.

Concentration (PHB 203):

Some spells require you to maintain concentration in order to keep their magic active. If you lose concentration, such a spell ends.

If a spell must be maintained with concentration, that fact appears in its Duration entry, and the spell specifies how long you can concentrate on it. You can end concentration at any time (no action required).

If the target is affected as if it was under the effect of Polymorph, (PHB 266), :

Duration: Concentration, up to 1 hour

The transformation lasts for the duration, or until the target drops to 0 hit points or dies.

then the duration relies on maintaining concentration. That duration is up to 1 hour based on concentration. In other words:

The transformation lasts for the duration, the duration is concentration of up to an hour or until the target drops to 0 hit points or dies.

The yuan-ti may choose to not maintain concentration in that case the transformation ends because maintaining concentration is one of the two conditions for a lasting transformation according to the Polymorph spell.

If it had a different duration, then the trait would explicitly say that the duration was different, but it does not - in fact, it explicitly says that the target is affected as if affected by the Polymorph spell with the restriction that it is transformed into a tiny Poisonous Snake (MM 334) and that the save DC is the same as that of the yuan-ti 's Innate Spellcasting ability.

The save DC is the same as that of the yuan-ti 's Innate Spellcasting ability.

Similarily the (unusual) ability Snake Antipathy:

Snake Antipathy (Abomination or Malison Only; Recharge 6). The yuan-ti targets a creature it can see within 60 feet of it. The target must succeed on a Wisdom saving throw or feel an intense urge to avoid snakes and snakelike creatures (including yuan-ti); as if affected by the antipathy effect of an antipathy/sympathy spell. The save DC is the same as that of the yuan-ti's Innate Spellcasting ability.

follows the duration logic that is tied to the Antipathy/Sympathy spell (PHB 214) which has a duration of 10 days and does not require concentration for maintaining the effect.

The Sticks to Snakes action-option of the yuan-ti explicitly states a duration in its description because the logic of the duration is not tied to the logic of the duration of a spell, it states explicitly:

The swarm remains for l minute, after which it turns back into the original materials.

This makes it explicit that Snake Antipathy doesn't require maintaining concentration and has a duration of 1 minute.


The alternative rulings.

  1. The same approach, The yuan-ti is not treated as if they are the caster: Polymorph into Snake takes 1 action. The yuan-ti is not treated as if they are the caster of a Polymorph spell. There is no caster that can maintain the concentration required for the Polymorph spell, so Polymorph into Snake immediately ends.

If you lose concentration, such a spell ends.

  1. A different approach, there is no duration (specified). There is no duration (specified), so Polymorph into Snake immediately ends.

The transformation lasts for the duration, or until the target drops to 0 hit points or dies.

  1. A different approach, there is no duration specified. The DM decides the rules, aka DM fiat applies. The DM will decide whether maintaining concentration is required and how long it will last (PHB 6):

Ultimately, the Dungeon Master is the authority on the campaign and its setting, even if the setting is a published world.

In the case of ruling three, read the first part of my answer for guidance.

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    \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – Rubiksmoose Jul 6 at 16:53
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The problem here is people mistaking the difference between Affect and Effect, and that you asked two questions instead of one.

Affect is a verb, it's something that is being done.

Effect is a noun, it's what is occurring.

Example: If I toast my bread, I'm affecting my meal by introducing heat to cook the bread to a desired state. The effect is that I have toast. The toast is the effect of what happened when I affected it with heat.

In the case of the Yuan-Ti ability, it states:

as if affected by the polymorph spell

Which means the Polymorph spell is affecting the target. That the ability specifically requires the target be turned into a Snake is simple specific > general rules. The important part is that everything within the spell Polymorph is applied to the target.

So, for your questions:

Does the Yuan-Ti need to concentrate on this ability?

This is going to require DM adjudication. This particular ability is located in the section detailed as Unusual Abilities, which are unique and weird abilities individuals possess that make them different from everything else. A caster not requiring to concentrate on a specific spell through the use of an ability would be a great example of this, and has a lot of mythological backing in the case of a Medusa. There are spells which have the same effect as her ability, although the way it affects a target are vastly different. The important point is that no part of her Petrifying Gaze ability makes mention to it affecting a target as if under the effects of any spell.

This is telling. To me, this says very clearly that when an ability references a spell, it's meant to be treated as that spell with any other specific over-rides detailed in the ability being used.

Since the Yuan-Ti ability makes no mention of Concentration being exempt, I would logically conclude that it is not.

Which leads me to your second question:

What is it's duration?

This part is actually answered in the spell Polymorph, which is currently affecting the target:

The transformation lasts for the duration, or until the target drops to 0 hit points or dies.

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  • \$\begingroup\$ Great answer, the specifc beats general rule is PHB 7 and the Action rules can be found in the MM 11 (recharge is on MM 12). \$\endgroup\$ – Akixkisu Jul 6 at 19:29

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