The doppelganger has the following traits and actions:
- Shapechanger: The doppelganger can use its action to Polymorph into a Small or Medium humanoid it has seen, or back into its true form. Its Statistics, other than its size, are the same in each form. Any Equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.
Multiattack: The doppelganger makes two Melee Attacks.
Slam: Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) bludgeoning damage.
If my adventuring party encounters a doppelganger who is shapechanged to look like, say, a sword-wielding human, how do I handle the doppelganger's actions in combat?
Possible Solution #1: Always use the slam attack.
The only attack listed in the doppelganger's stat block is the Slam. This seems like an effective attack, but if I were a player and I noticed that an orc attacked me by "slamming" my character instead of using its greataxe, I'd be suspicious that something fishy was going on. So another possible solution would be...
Possible Solution #2: Use the weapon typically used by the creature it is shapechanged into.
The doppelganger doesn't have a typical weapon attack in its stat block, which I assume is because most doppelgangers won't typically be equipped with weapons (in the way that orcs are generally equipped with greataxes and javelins). However, I also know that just because you shapechange doesn't mean you gain or lose any proficiencies, so I don't think it's safe to assume that a doppelganger can just use whatever weapon is typically used by whatever it's been shapechanged into. Moreover, unless the doppelganger uses an ammunition or finesse weapon, its attacks are going to be way less effective (since its Strength modifier is 0 and its Dexterity modifier is +4).
So which is it? Use the Slam for a more effective and suspicious attack or use the "disguise's" weapon for a less effective but non-suspicious attack?