Detecting doppelgangers on a large scale

Question

How could a warring faction reliably detect doppelgangers that are used on a large-scale by their enemy?

Say that two modestly-sized military powers are at war. One of the sides manages to militarize doppelgangers, using them en masse for covert operations (spies, assassins, abductions, etc) on a scale that they are not traditionally seen on (say in the order of hundreds of doppelgangers). How does the opposing faction defend itself from these infiltrators?

This question differs from the typical question of how to detect doppelgangers due to this issue of scale. While most doppelganger conflicts are "one and done", in which the problem will be solved once the doppelganger is detected and dealt with a single time, solutions here need to be reliable and repeatable. It should be assumed that the doppelganger faction will adapt to tactics that are repeated too often or are otherwise predictable. Doppelgangers will also continue to reinforce themselves perpetually over the course of the war, preventing even the most effective one-time solutions from solving the problem permanently.

Answer 1

I suggest that the side without the doppelgangers consider investing on the creation of Lanterns of Aura in bulk (which cost 1,000 gp to create). These items are simple enough so most 3rd level casters can make them, and are even easier to use, not requiring any special knowledge unless you want to actually identify the magic auras.

Those could be stationed at guard gates or at the entrance of any important locations, and anyone who has a magic aura should be stopped for magical inspection, similarly to what we see at airports metal detectors (though those do not detect doppelgangers). If people are carrying magical equipment, they will have to remove everything magical before proceeding.

Another solution would be producing Rings of Arcane Signets (500 gp to create) for all officers. They produce an unique sign that will not change between people. Those are harder to create because they require the Forge Ring feat, but they are much safer as each person would have a specific image/symbol related to them, which could be identified by mundane characters.

Any other form of magical identification on items is prone to failure due to spells like Magic Aura, which can mask magic items as mundane or as a different type of magic item.

Lastly, the Arcane Mark could be used to identify all personel under your army, if you have enough people to either put their mark on other's skin or on identification badges. This mark is unique to each spellcaster (similar to the ring of arcane signets), can be made visible or invisible to mundane sight, and can't be easily duplicated by spells, even if spells are easily available to the doppelgangers.

If you combine both the lanterns and the arcane signets, officers would be easily identified as who they say they are and could skip the inspections on the Lantern Stations spread across the city. Doppelgangers would have to smuggle themselves into the city and between districts and avoid important buildings altogether. If they happen to steal an arcane signet, their sign would be different from the original person, so they would have to be inspected and identified as an enemy shapeshifter.

Another thing to keep in mind is that Doppelgangers are Monstrous Humanoids, and not Humanoids. So any spell and special abilities that work specifically against Humanoids, will not work on them. And those that work specifically against Monstrous Humanoids, will suddenly work on that obviously-not-a-shapeshifter-noble trying to get past the guards.

This, obviously, includes many spell effects that specifically target humanoids, like Enlarge Person or Charm Person. But also includes not so obvious spells, like Daze (effectivelly, Daze Humanoid). Daze obviously has a limit of 4HD, but this is a method that can be applied to most of a city's population, and if they resist even if being told not to (which the caster would immediatelly know), they certainly deserve an inspection. The spell can be made available to non-spellcasters through the Cloak of Hedge Wizard (1,125 gp to create)

Items like the Cap of Human Guise (400 gp to craft) would also instantly fail to work when used by a monstrous humanoid, as the item can only be used by small or medium humanoid creatures. The doppelgangers could adapt themselves to copy the illusory form, which would be immediately detected by spellcasters.

Feats like Shapeshifter Foil are very good at the hands of inquisitor-type characters, though they require at least a few levels of a spellcaster class. But a single point of damage should be enough to reveal a shapeshifter.

These options based on creature type will simply fail if the doppelgangers obtain the Human Guise feat though. But at least your are forcing them to be trained against (some of) your security measurements.

Answer 2

Tie detect magic to hallow

The usual way to defend structures and more permanent camps is with the spells hallow or unhallow. In addition to other effects, these spells allow tying to an area a particular spell, and one of the spells that can be tied to a hallow or unhallow spell is the spell detect magic that can conveniently detect magically disguised doppelgangers. A cleric 9 can cast the spell, and, while the effect must be renewed yearly, in the mean time its duration is instantaneous (therefore immune to attempts to dispel it) and the material components run only 2,000 gp for the combined hallow plus detect magic effect, so this is unlikely to deplete the treasury even if the war runs long.

It takes going back to 3.5e to figure out some of the details of how some hallow-tied spells work, and, even then, how exactly a detect magic spell works when tied to a hallow spell remains unclear. This GM suggests that on the round the creature enters the warded area, anyone viewing that creature discerns whether magical auras are present or absent on that creature. On the second round after that creature's entered the effect, those who saw that creature enter can discern the number of different magical auras present on and with that creature and the power of the most potent magical aura. And, on the third round after that creature's entered the effect, those who saw the creature enter can discern the strength and location of each aura (and skilled onlookers can learn more about those auras—see the detect magic spell). It'll make things easier for all involved, of course, if creatures that enter shed any magic items they're carrying and reveal if they're aware of any ongoing spells that are currently in effect on them so that an expert need not be called. (Putting a hallow effect that's tied to a dispel magic effect in an nearby location may also prove useful—while this combination won't end the doppelganger's supernatural ability, because it won't do that, this may reveal the creature as a disguised doppelganger!)

After such areas are established, it's just a matter of the folks in power establishing how (ahem) arcane, draconian, and paranoid the security measures surrounding the detection process should be… and determining how much inconvenience the populace is willing to tolerate. This is left as an exercise to the individual GM. (This GM is unfamiliar with the asker's campaign setting.)

Yes, this essentially creates an contemporary airport-style check-in for the location, camp, or town. And, if the location's a chokepoint that's used for a town's ingress and egress, everyone coming and going from the town could be scanned for magic at 18-second intervals. (Citizens present inside the town prior to this checkpoint's establishement should also eventually be brought to the location and examined so as to weed out thoroughly embedded doppelganger spies.)

Combined with lanterns of auras (as per this fine answer) and cloaks of the hedge wizard (divination) for more subtle counterespionage, not only can doppelgangers be detected, but magical resources in general can be enumerated… valuable information for any facing a fantasy war.

Answer 3

I'm going to assume you want something that works quickly and effectively for mundane characters without recourse to any magic. Personal security questions are the only methods I know of that can give you reasonable prove against infiltrators. People check that someone is who they say they are by asking a question about an experience that occurred in their shared history, this only directly works one-on-one but you can extrapolate to large numbers relatively easily.

Answer 4

Depends on how many mid-level clerics they have. A tenth level cleric can cast Commune twice per day for a cost of 500gp per casting. If you split the army into fractal segments of halves and the army was 20,000 men strong, ten questions (or a single casting) could narrow it down to twenty people to individually check for being imposters. Of course you'd probably want more castings if you want to actually get a significant number of them and you might not be able to pay for that. Assuming they only have 500 doppelgangers total, with no more than one to a squad of 20, it would cost 250,000gp to track them all down and up to 250 days to do so depending on how many level 10 clerics you have. Higher leveled clerics can cut down on the time and cost, as they can ask more questions per casting and cast more times per day. A single 20th level cleric could do it for 62,500gp in 63 days, assuming they didn't use any higher level spell slots. Of course you may want to use these spell slots for doing other information gathering missions which would slow everything down.

Answer 5

The doppelganger's main advantage being to deceive eyesight and hearing, an identification based on some other senses, combined with the other answers, could be effective. For example, having a badge or some kind of item which would be infused with a special scent allowing the identification.

The doppelgangers could be able to take the appearance of a commander, but could not imitate the scent of his/her badge. The solution remains vulnerable to the steal of these items, but this vulnerability is not directly relative to the capacities of the doppelgangers anymore. With a special and well protected recipe, this could prevent the identity theft of the army leaders or allow a more efficient access control.

This solution should be less usefull to identify all the soldiers of the army, the probability of a badge beeing stolen becoming too high. However, a regular change of the scent or having a scent depending of the rank could help to prevent the vulnerability of theft/reproduction.

On a larger scale and without the theft issue, perfume each army member with a secret composition scent should also counter the doppelgangers abilities.

Answer 6

Repeated use of one-time methods is by far the most efficient solution

I realize you are worried about scaling, but most of the answers in the linked question have no real issues with scaling. Detect Magic, for example, is a 0 level spell that works 100% of the time unless the doppelgangers all have class levels in Witch (or occultist or psychic, with the right specializations).

Even if they do, a single casting of e.g. Youthful Appearance (a harmless 1st level transmutation(polymorph) spell usable on creatures other than the caster) for a wide variety of classes) will always reveal the doppelganger, no matter what. This is because even if the doppelganger is reading the mind of the caster, has a readied action to change shape, the caster fails the spellcraft check to recognize the polymorph spell as it's cast, and the DM generously rules that dismissing its current shape in order to accept the effects of the new spell transitions the creature smoothly into the new form without observers noticing the true form obtained in the middle, change shape grants the doppelganger the actual abilities of its form while Youthful Appearance (and, like, every other polymorph effect short of polymorph any object) does not. That means simple mundane tests can be devised and distributed in kits to discriminate between highly-adapted doppelgangers and persons not under the effects of Change Shape.

And that's all assuming the Doppelgangers are using magics of some sort or another to have a chance of getting around the Detect Evil/Chaos spam that's quite prevalent anywhere Paladins are found.

Basically, the resources needed to check an individual or group for doppelgangers are much, much less than the resources the side supplying the doppelgangers must spend to counteract each surveillance measure, even with the standard once-and-done methods. Therefore, it's unlikely to be worth employing any methods beyond the standard ones.

As a side note, if your anti-doppelganger side has a Slayer, that slayer has the Blood Reading talent (it's one of the only good 2nd level slayer talents, and it's a unique ability that only slayers can get, ever, so every slayer of at least 2nd level should have it). That talent lets you non-magically know the exact hit point total of any living creature you can see with a move action, which is sufficient to discern any doppelgangers who are mimicking any creatures with more hit points then they have (unless your DM rules that Blood Reader detects current hit points+temporary hit points and can't tell the difference AND the doppelgangers have the exact right amount of temporary hit points), and requires the doppelgangers to self-mutilate to mimic creatures with fewer hit points than they have.

If your anti-doppelganger group has a gunslinger of at least 4th level, they can also take the Run Like Hell dare, which makes them go from 0->1 grit whenever an enemy isn't within 100 feet (and, correspondingly, lets them know whenever an enemy is within 100 ft if they bother to check). In combination with a deed like that from the Secret Stash Deed feat, this check can even be done discretely and as a non-action (and thus usable out of turn). This doesn't find doppelgangers, it just finds any enemies no matter what, which happens to include doppelgangers that aren't on your side.

Beyond all of that, though, there's probably no real reason to think that doppelganger infiltration is a serious threat, and thus probably no real reason to attempt to combat it. A doppelganger possesses no particular defenses against being persuaded of the righteousness of your cause, and so presumably changes to your side as soon as it gets within range of your leader's Convincing Lie ability or is otherwise subjected to your society's homeostatic Bluff/Diplomacy background. It's worthwhile to take precautions against the impersonation of influencial or important persons, but there's probably no reason to deter your opponent from giving you their valuable doppelganger sleeper agents-- anyone engaging in foreign social contact without special tactics and abilities is very unlikely to end the day the same person they started it as, such is the nature of Pathfinder.