Nondetection says it hides the target from all divination spells. Invisibility hides you from normal sight. When combining both effects on one target, are they hidden from true seeing?
The beneficiary of a nondetection spell:
- Can’t be targeted by any divination magic, and
- Can't be perceived through magical scrying sensors.
There are only two spells in the PHB that create magical scrying sensors: clairvoyance and scrying. These are obviously covered by Item 2.
It's reasonable to assume that the remaining 28 spells from the divination school in the PHB, including true seeing, are covered by Item 1. They certainly fall into the category "any divination magic," so the question is, what does it mean to be "targeted" by these spells?
Does the spell have to explicitly refer to a "target" in its spell description? That's only two spells: the cantrip true strike and the 1st level ranger spell hunter's mark. This would be a very short list for a 3rd level spell, and neither of these is really a 'detect'-class spell. We need a broader definition for "targeted" than this.
It might be tempting to equate the target of a spell with the range of a spell; however, at least in the case of divination spells, this yields rather absurd results. Out of the 28 spells, 17 have a range of "Self," while another 4 have a range of "Touch" where the spell description specifies a willing creature. Clearly the recipient of these spells is not the one being "targeted" by them. Otherwise this would imply that a caster under the protection of nondetection, for instance, would not be able to cast spells like comprehend languages or speak with animals on themselves for the duration. That is clearly not the intent of the spell. Furthermore, even if a creature thought this was a desirable effect, they could avoid being "targeted" by 3/4ths of all divination spells simply by not casting them or by declaring that they aren't "willing."
In addition, if we assume that the target of a spell with range "Self" is the caster's own self, and not a creature one is trying to detect, then nondetection would provide no protection from detect evil and good or detect thoughts, which would seem to be exactly the kind of thing for which this spell is intended.
There is even a case to be made from RAW that target is not the same as range. Consider detect thoughts. It has a range of "Self," and it allows the caster to detect creatures they can't otherwise see. Yet the rules for Targets say (PHB 204): "Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature’s thoughts, typically goes unnoticed." So the range of the spell is the caster's self, but the target of the spell is the creature having its thoughts read.
Since "target equals range" leads us to absurdity, I propose a simpler, alternative reading of the spell: the recipient "can't be detected by any divination magic or perceived through magical scrying sensors." If they can be detected by other means, fine; but if the only reason they can be detected is because of a divination spell, as is the case with an invisible creature who could be perceived with true seeing but is otherwise hidden, then they remain undetected.
For the duration, you hide a target that you touch from divination magic. The target can be a willing creature or a place or an object no larger than 10 feet in any dimension. The target can’t be targeted by any divination magic or perceived through magical scrying sensors.
This spell gives the willing creature you touch the ability to see things as they actually are. For the duration, the creature has truesight, notices secret doors hidden by magic, and can see into the Ethereal Plane, all out to a range o f 120 feet.
RAW about targets
Targets A typical spell requires you to pick one or more targets to be affected by the spell’s magic. A spell's description tells you whether the spell targets creatures, objects, or a point o f origin for an area o f effect (described below). Unless a spell has a perceptible effect, a creature might not know it w as targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature’s thoughts, typically goes unnoticed, unless a spell says otherwise.
The target of trueseeing is the one that receives it's power, so the first part of nondetection isn't working(can’t be targeted). True seeing is indeed a divination spell, but it grants truesight, that is not a magical scrying sensor. Thus the second part of nondetection doesn't work either.
A creature with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceives the original form o f a shapechanger or a creature that is transformed by magic. Furthermore, the creature can see into the Ethereal Plane.
Yes and No
I think the issue here is that people are confusing True Seeing with Truesight. Non-detection absolutely does make a creature immune to the True Seeing divination spell. It does not make it immune to the Truesight inherently part of a creature or [up to the DM] the benefits granted to another creature that has been targeted by a True Seeing spell. While a DM allowing Invisibility and Nondetection to work together like this doesn't break the game at all, the interpretation of the wording as such does have ramifications on other similarly worded spells such as Greater Invisibility.
If all similarly worded spells stacked as some would like, Greater Invisibility and Nondetection (4th and 3rd level spells) would give creatures like Empyrean (MM 130) with CR 23 almost no chance against low level adventures in many combat situations and would break entirely the deadly encounter XP and CR balancing tables of DMG on page 82.
It is important to note even the highest rated answer has now been edited to acknowledge that only Truesight granted by a divination spell such as Trueseeing should be considered to fail. Truesight inherent or granted by any other means (such as an item) still works to detect the invisible creature.
Other examples of circumstantial evidence on related matters:
Mind Blank also grants "one willing creature you touch immunity to.. divination spells..." and like Invisibility and Greater Invisibility does not grant immunity to Truesight. Many similar spells also distinguish this explicitly. Most would also agree that having a blank mind and being invisible, despite identical wording, wouldn't make sense to also all grant immunity to Truesight, but certainly would prevent scrying and other such things that Nondetection prevents.
Clairvoyance (222 PHB) also starts with "You create an invisible sensor"... then explicitly states "A creature that can see the sensor (such as a creature benefiting from see invisibility or Truesight) sees a luminous,.." Non-Detection does not remove Truesight from creatures and so creatures with Truesight can see the sensor, however a creature affected by Nondetection can't be seen by the sensor in return.
As a historical precedence (since most who have been around since 2e recognize the lore (and spell list) of 5e as a direct import from 2e): In second edition "Improved Invisibility" and "Spell Immunity: Divination" was a well know exploit. However, even back in 2e, a different spell called "Non-detection" did not protect against true seeing, as it was designed (and still is designed) to protect against scrying and other divination magic that targets the invisible creature. One can compare the wording to see the differences of the intended functions of these spells in 2e then compare to the modern 5e wording to gain historical insight.
Up to the DM ultimately to be the judge on what breaks the game play or not.