Invasion of the body snatchers! Doppelgängers are shapeshifting humanoids (though the fifth-edition Monster Manual categorizes them not as humanoids but as monstrosities) that take on the appearance of other beings for fiendish purposes.
Doppelgängers have high Constitution and extremely high Dexterity, making them scrappy fighters. Their self-preservation instinct is strong, and they’re unusual among monsters in having a high Charisma, along with proficiency in Deception and Insight. They can’t be charmed, they can Read Thoughts, and they have the Ambusher and Surprise Attack features in addition to their Shapechanger power. All these abilities synergize to make the doppelgänger a sucker-puncher par excellence.
It begins with the disguise: a doppelgänger has to take on the semblance of someone who’s beyond suspicion, or at any rate one whom the player characters are likely to underestimate. Read Thoughts is an unopposed ability that lets the doppelgänger lock on to anyone within 60 feet and scan its “surface thoughts” (i.e., whatever it’s thinking at the moment), granting it advantage on Deception, Intimidation, Persuasion and Insight checks against that target. Being proficient in Deception, it will favor this approach in particular, though it won’t eschew Persuasion or Intimidation if the situation calls for one of them. To put it in more transactional terms, it’s more inclined to bluff or seduce its way toward its desired goal than to rely on negotiation, pleading, demands or threats. It may also exploit its Insight to shame someone into behaving a certain way, or to insult that person in order to gain favor with others present.
But the key is this: The doppelgänger doesn’t want to fight if it can help it. As long as it can advance its interests without fighting, relying solely on misdirection, that’s what it will do. And if it does have to fight, it certainly doesn’t want its opponent to be the one who starts the fight. It attacks when the opponent has no reason at all to expect it to.
The doppelgänger, you might say, has more EQ than IQ. It chooses its target not according to who poses the greatest tactical threat but who poses the greatest social threat—the one most resistant to its deceptions, which will generally be a character with a high Wisdom or one who’s shown suspicion of the doppelgänger from the start. In fact, a blunt “I don’t believe you” or “Who are you really?” may be the spark that sets the doppelgänger off.
Ambusher gives the doppelgänger advantage on attack rolls when its target is surprised; Surprise Attack gives it an extra 3d6 damage on a first-round surprise attack. It gains these attack and damage bonuses for the entire first round of combat, meaning it can apply them to both of its Multiattacks. Two attacks each dealing 4d6 + 4 bludgeoning damage—an average of 18 points per hit, 36 if both land—can take out one or even two low-level player characters before anyone has a chance to react.
Unfortunately for the doppelgänger, though, that first-round nova attack is its only trump card. After that, it has no way to regain the upper hand in combat, although it does still enjoy two attack rolls per action and a large reservoir of hit points. (This is much less advantageous once the PCs have reached level 5 and gained Extra Attacks of their own.)
Once its enemies are no longer surprised, it’s torn between the desire to finish them off and the desire to get away and hide in a new form. If it seems to be winning the fight, it won’t flee until it’s seriously injured (reduced to 20 hp or fewer), but if it’s obviously going to lose, it will commence evasive maneuvers when only moderately injured (36 hp or fewer). In a more ambiguous situation, it will make the decision somewhere in between.
One thing it absolutely doesn’t want is to let itself be surrounded. If engaged by three or more melee attackers, or flanked by two or more, it will Disengage and reposition itself so that (a) it can face all its enemies at once, and (b) it’s closer to the exit.
Otherwise, it will Attack (action) whoever’s engaging it, then use its movement to reposition itself within the opponent’s zone of control so that it can flee if it needs to. When retreating, doppelgängers always Disengage (action) to avoid opportunity attacks.
There’s a good chance that combat with a doppelgänger will be followed by a chase (see Chases, pages 252–55 of the Dungeon Master’s Guide), because no one wants to let a doppelgänger get away and disguise itself again. Of course, this is exactly what the doppelgänger wants most. Ideally, it wants to vanish into a crowd, but reaching a less populated location where it can transform itself into someone who seems to belong there—a farmer in a field, say, or an angler by a lake or stream—is the next best thing, as long as it can make its transformation out of its pursuers’ view. It takes one action for the doppelgänger to transform, so it needs to have enough of a lead to go unseen for a full turn.
Nezznar’s minions in the collapsed cavern in Wave Echo Cave (part 4, room 18) include a doppelgänger calling itself Vhalak, which will try to flee to room 19 if its side is losing a fight. It then disguises itself as Nundro Rockseeker, the brother of Gundren Rockseeker held prisoner by Nezznar. The idea is that Nezznar uses his “hostage” (Vhalak) to compel the party to surrender. But when I ran this adventure, things went somewhat differently: The PCs went to room 19 first and fought Nezznar, his bugbear henchmen and his giant spiders before ever visiting room 18. Vhalak came running at the sound of battle but fled again when Nezznar was killed, and the PCs were too occupied to pursue. The doppelgänger then disguised itself as a beaten-down Nundro and let the PCs find it in the hallway outside room 20—the room in which the real Nundro was being held prisoner! The PCs were in bad shape after the fight with Nezznar, so they opted to take a long rest in room 19 before proceeding. That didn’t go well for the party’s sorcerer, who took the middle watch and got clobbered by the doppelgänger while everyone else was asleep.