Qfyilyi

How powerful would an always-on passive ability or magic item or whatever else that could grant truesight to a PC be? Let's say 60 ft. to match the range of races with Darkvision.

For reference, here's what truesight does (MM, p. 9):

A monster 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 perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the monster can see into the Ethereal Plane within the same range.

On the one hand, the spell true seeing (PHB, p. 284) is a 6th level spell, which is pretty powerful, and only grants truesight for an hour per casting (although it does have an impressive 120 foot range):

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 of 120 feet.

On the other hand, a Monodrone (MM. p, 224), which is only a CR 1/8 creature, has truesight, as do quite a few other "weak" monsters, which implies that it's not that powerful.

So what should an always-on truesight ability be compared against to determine how powerful it is? It's clearly superior to Darkvision, which some (most) races start off with. Is it on par with a legendary magic item, or is it less powerful than that? A feat, or is it more powerful than that? I can't think of any other ways to grant a PC a passive always-on ability besides feats or magic items (outside something like a Blessing or something...)

That's a 20th level epic boon (so very powerful)

The power of a permanent truesight effect would be situational. If you measure 'power' by raw combat ability, then unless you're fighting invisible creatures truesight has little effect. Even if this player is fighting invisible monsters, the rest of the party is still vulnerable. So it shouldn't break combat balance. Outside combat, truesight would also be of little use if you don't encounter illusions or shapechangers or ethereal creatures, although if you do encounter those things it solves those challenges trivially, which may be an issue for the DM trying to make interesting challenges. The range would also be a big factor. Truesight to a range of 10ft (for instance) requires the player to be standing very close to the item of interest and wouldn't be as powerful as a range of 60ft, which would reveal an entire room.

But if we want to see how powerful the ability of permanent truesight is in general, we can look at where the game designers have made truesight available for player characters. You have already noticed truesight's availability as a 6th level spell, which automatically puts it in a rather high echelon of power. But where would permanent truesight fit into the picture?

The answer is found in page 232 of the Dungeon Master's Guide, under Epic Boons. Epic Boons are only available to player characters who are at level 20 and are made to extend the power of already-powerful 20th level characters. Of these epic boons, there is one which provides exactly the effect you are considering:

Boon of Truesight

You have truesight out to a range of 60 feet.

If the game designers thought that permanent truesight (to 60ft) is something worthy for only a 20th level character, you should be very cautious with giving out a similar ability to a lower level player character. Putting a much shorter range and/or limited uses per day might (might) help balance it for lower level characters, although it will still be very powerful in certain situations.

Truesight is a fairly powerful ability for a PC that has the undesirable effect of limiting the DM's ability to use certain types of storytelling. The ability to simply ignore effects like invisibility, illusion, or darkness makes it almost impossible to conceal information from the players, and the ability to immediately see through transformations tends to remove several whole classes of story from the DM's arsenal. That, in general, means the game becomes less varied, less interesting, and less exciting. In exchange, one character gets to feel cool once in a while by telling the DM, "Nah, that effect doesn't matter to me."

There are many abilities that are very powerful in PCs' hands but easily accessible to certain monsters. This doesn't mean the ability is weak; it means the PCs are going to be limited in some specific way for the duration of that one fight or encounter. Most monsters have a very brief moment in the spotlight (whether as an opponent, a non-combat encounter, or an ally), and then the PCs move on. Even an ability that turns the whole game system on its head is only "just for now" in the great scheme of things. An ability that disables one or more characters' specific shtick is only a sign that says "Your usual strategy won't work here, try something new!" But next time, you can go back to what you normally like doing.

Hand that over to a PC, though, and suddenly the whole game is permanently on its head, because a PC is always there, always involved, and always has the ability.

If you meet a monodrone (or several), they're there for the duration of one fight, or at most an extended adventure. Their truesight gives the modrons a specific, intentional flavor -- they're made out of Law and can't easily be fooled or bypassed. But that Gnome Illusionist isn't totally and permanently worthless; next session you'll be on to some other adventure, and those illusions will be nice and powerful again.

Similarly, a number of low level enemies have wide-ranging resistance or immunity to damage and debilitating conditions -- the Helmed Horror springs to mind. As a single fight, it's just a challenge to figure out how to get past the limitation. Give those same resistances to a PC, and they're game-breaking.

The true seeing spell is a really good signpost in this case. True sight is so strong that one of the few ways for PCs to ever get it is from a high level spell -- one that most casters can only ever use once per day and which competes for space with powerful effects like chain lightning, disintegrate, and flesh to stone. Right away, that should tell you how the designers feel about this ability in the hands of a PC.

Short answer: very powerful

You, as a DM, would essentially take out a big part of your "toolkit"; you could no longer be able to use invisble opponents, illusions to hide or trap anything, things that rely on the ethereal plane to hide themselves (ghosts, etc).

On an insignificant monster like the Monodrone it is not a very powerful ability - but it is not because of the ability itself, it is because of the Monodrone which would not pose much of a threat even if you gave it a +5 vorpal blade (so to speak).

Consider a vampire, high level assassin or a demon with Truesight and maybe you can see the difference vs a monster that poses no real threat to any higher level adventurer.

Very powerful

As per DocWeird's answer, giving a PC Truesight immediately removes a large swathe of potential encounters from your DM toolkit. You will no longer be able to use illusions that conceal secret objects or shapechangers disguised as town mayors and so on. You will essentially be shooting yourself in the foot.

I wanted to add my own answer though just to point out that you cannot compare monster abilities with PC race or class features on a like-for-like basis.

Monsters tend to be one-off "disposable" creatures as opposed to PCs that have (hopefully) a continuing existence. Giving some magical ability to a monster may not have too much of an effect because, realistically, they will only get the chance to use it once or twice before they get slain (or defeated and never seen again). So even a low-level monster could have a "powerful" ability without unbalancing a game.

Whereas the same ability in the hands of a PC, who can use it all the time, against every monster...suddenly the relative 'power' of that ability becomes magnified several times over.

the truesight ability would work like normal allowing for the PC to see through most magic things.