Does Faerie Fire affect objects/creatures partially in the area of effect?

Question

What effect does Faerie Fire have on objects/creatures partially within its 20' cube area of effect?

Two simple examples: a 10' pole which is half-in, half-out of the area of effect, or a purple worm that is partially within the area of effect.

I'd say Faerie Fire exemplifies some the the complexity of adjudicating spell effects on larger objects/creatures. The spell description describes a 20' cube area of effect. However, if objects/creatures partially inside the AOE light up in their entirety, then the spell effects could extend outside the 20' cube. If the whole object lights up, someone could make a signal wire -- long (100'+) wire from point A to point B. Caster at point A casts Faerie Fire, signal shows up at point B instantaneously.

In general, I'm unaware of any rules/rulings on how much an extended object/creature needs to overlap with a spell's area of effect for them to incur the spell's effects. Some spells clearly indicate that the full object/creature needs to be completely within a specified space to be affected. But this spell does not.

Answer 1

Partially within an area is within the area

Faerie Fire says:

Each object in a 20-foot cube within range is outlined in [...] light. Any creature in the area [...] is also outlined...

So if the creature or object is in the area, then it is affected, outlined entirely by the faerie fire effect.

Does a creature or object have to be fully in the area to be affected? Definitely not. That isn't how spells are ever expected to work. For example, if an ogre is only partially in the area of a fireball spell, our expectation is that the ogre is affected and takes full damage. No rule suggests that it would take damage based on what percent of its body is inside the spell's area. Conversely, if an ogre that occupies 4 squares is inside a spell's area, we expect it to take damage once only, not four times. Either it's a target and it's affected like a target, or it's not and isn't.

Where is it written? The exception that proves the rule

There doesn't seem to be a clear rule anywhere that actually says that "partially within the area is still within the area", but it can be seen by when there is a stated exception.

As an example of this, the stinking cloud spell says:

Each creature that is completely within the cloud at the start of its turn must make a Constitution saving throw against poison.

Why does it specify 'completely within the cloud' rather than just saying 'in the cloud'? The reason is because normally being partially in an area counts as being in the area, and this particular effect is an exception where it has to completely engulf a creature to affect it.

Similarly, thunderwave specifies that

unsecured objects that are completely within the area of effect are automatically pushed

...which is only necessary if "objects that are within the area" would normally include things that are even partially in the area.

The exception proves the rule; the existence of an exception is proof that an unstated rule exists.

What are the limits?

So this brings up the question of how big an object is. Is a castle an object? Is a ship an object? If you straighten out a rope, can the whole thing glow if you only catch one end in the area? If a coiled rope is in the area, and then you uncoil it, does it keep glowing?

These are questions a DM will have to grapple with, but in general the DMG says that "an object is a discrete, inanimate item [...], not a building or a vehicle that is composed of many other objects". So a ship or castle should not light up from a single spell.

I see no harm in allowing that a rope lights up if you hit one end of it with faerie fire, but if you as DM find that there's some trick the players are trying to exploit, it's within your realm to declare that it doesn't work that way.

Answer 2

To be affected, it should suffice if any part is in the area

The rules never clarify how much of a creature or object must be exposed to a spell's area of effct for it to be affected by the spell.

The rules for Area of Effect (PHB p 201) only talk about when a location is in or not in the area, but not how much of a creature or object needs to be exposed to be affected. When you play using the optional rules for a square grid (DMG p. 251) the advice is that "If an area of effect is circular and covers at least half a square, it affects that square.", but that does still not say anything about what happens if only part of a creature or object is in that square. The sections about creature size or object size in the core rules have nothing either.

So, strict Rules-as-Written, this is not defined, and up to the DM to decide.

However, we have a clear statement on designer's intend of how this is supposed to work. Fireball also has an area of effect and states

Each creature in a 20-foot-radius sphere (...) must make a Dexterity saving throw.

So, when is a creature in that 20-foot radius? If any part of it is in the radius? If more than half of it is in the radius? If all of it is in the radius? Here is what Jeremy Crawford has to say on it:

Fireball gets you no matter how much of you is in the fire's radius. #DnD

While not an official rule, the designer intent is clearly that it is sufficient of any part of the creature to be exposed, for the the creature to be affected. By analogy, the same would apply here. If any part of the object is in the cube, it will be affected by faerie fire.

Answer 3

Yes, partially ...

While I am aware that this question is already tagged answered, and also many may prefer a generally valid answer, I want to offer another answer. To keep it simple, and only for this spell, I rule it this way:

As many answers already constitute: Creatures (and objects) are affected by this spell even if they're only partly in the area of effect: Attacks are made with advantage: It does not depend on the size of a creature, because you only attack a single square (melee or ranged combat), anyways.

But only those parts of objects and creatures within 20 ft. are glowing dimly.

The moving of creatures also results in changing parts of their bodies that glow during the spell duration. These changes may also affect creatures that enter the area later within the spell duration (DM decision). But the RAW spell description reads:

(...) Any creature in the area when the spell is cast is also outlined in light if it fails a Dexterity saving throw. For the duration, objects and affected creatures shed dim light

... so keeping strictly to the written rules: 'New' creatures in the 20 ft. cube won't be glowing. But also RAW: new objects (perhaps thrown into) would glow (note the missing "affected" in front of "objects" in the second sentence)

In my interpretation this spells alters the reality in the 20 ft. cube, in such a way that it lits up the included things (creatures and objects). That way, the target of the spell is not the group of creatures and objects, but the space itself. Hence, the ability of dynamically changing glowing parts of bodies.