What would happen if you cast Simulacrum on a shapechanger?

Question

While there are many kinds of shapechangers in the D&D universe, such as vampires, lycanthropes (all of them), imps, doppelganger and changelings, there isn't much said about how they interact with the Simulacrum spell.

The Simulacrum spell states:

You shape an illusory duplicate of one beast or humanoid that is within range for the entire casting time of the spell. The duplicate is a creature, partially real and formed from ice or snow, and it can take actions and otherwise be affected as a normal creature. It appears to be the same as the original, but it has half the creature's hit point maximum and is formed without any equipment. Otherwise, the illusion uses all the statistics of the creature it duplicates, except that it is a construct. [...]

Does this mean that a polymorphed being (be it from spell, curse or natural ability) would be copied in their current form? or would their "true form" emerge from the snow?

I can see this being quite useful either way:

• Your party's changeling studies and creates a persona of the king or an important person, then you create a simulacrum of the changeling king. Now you have 2 or 3 walking talking kings to create all sorts of chaos.
• Though it would take 12 hours, if Simulacrum would reveal a true form, one could easily prove who is the real king by simulacruming one of the copies.

Answer 1

It's up to the DM.

I can't nail down an adequate RAW interaction between simulacrum and a creature that has had its form changed through (true-)polymorph or some other method. I will give how I would rule it with my reasoning. Without exploring all the details of all the possibilities for shape changing and how they interact with simulacrum, this is the best I can do for general guidelines.

1. I would rule that the simulacrum takes the form the target had at the end of the 12 hour casting time.

This tweet from Jeremy Crawford affirms this ruling. This is, of course, not an official ruling, but I am not claiming my ruling to be RAW or official at all. This seems to be the best objective way to rule it.

2. The simulacrum is still a copy of the original creature, with its shape changed.

This seems clear enough from the description of simulacrum:

the illusion uses all the statistics of the creature it duplicates.

I would rule that the true form of the original creature is baked into "all the statistics of the creature".

3. If the original creature had its form changed by a spell requiring concentration, such as (true-)polymorph it immediately reverts to the original creature's true form.

Because the simulacrum is not the target of the concentration spell, the magic holding its shape is not holding its shape. It reverts to the original creatures true form. Note, I would argue that this portion of my ruling is a RAW ruling.

4. If the original creature had its shape changed by more permanent means (such as a curse or natural ability), the shape changed form is maintained, and can be changed by the conventional means attendant to the change of form of the original.

Again, this is, I think, baked into "all the statistics of the creature".

Answer 2

It depends on the creature.

As you quoted, the Simulacrum spell states:

You shape an illusory duplicate of one beast or humanoid that is within range for the entire casting time of the spell.

Simulacrum goes on to say:

It appears to be the same as the original, but it has half the creature's hit point maximum and is formed without any equipment. Otherwise, the illusion uses all the statistics of the creature it duplicates, except that it is a construct.

Non-Humanoids

Both Imps and Doppelgangers have the following line in their Shapechanger ability:

Its statistics are the same in each form, except for the speed changes noted.

Creature type is part of their statistics; even if they look like a humanoid or beast, their creature type hasn't changed making them invalid targets for simulacrum. The spell would simply fail, which is telling in and of itself.

The vampire's bat form has a similar line. The mist form is clearly neither beast nor humanoid.

Changelings & Lycanthropes

Changelings and Lycanthropes are humanoids, a valid target, and would be duplicated. We have to look to the features that grant them alternative forms to see how they interact.

Change Appearance. The changeling can use its action to polymorph into a Medium humanoid it has seen, or back into its true form. Its statistics, other than its size, are the same in each form.

Each Lycanthrope's shapechanger property has slightly different wording, because some of them change size or AC, but they all indicate something to the effect of.

Shapechanger. [...] Its statistics, other than its size and AC, are the same in each form.

The statistics don't change when it uses change appearance, so the simulacrum would still be a changeling or lycanthrope.

Druid Wildshape

It wasn't part of the initial question, but it's worth noting that the Druid class' wildshape ability is worded differently. A druid's statistics are replaced, so it's type actually does change - it really is a beast.

Your game statistics are replaced by the statistics of the beast, but you retain your alignment, personality, and Intelligence, Wisdom, and Charisma scores.

Polymorph & True Polymorph

Polymorph is a non-issue, because it only lasts an hour and simulacrum takes twelve hours to cast.

True Polymorph is a thornier matter.

If you concentrate on this spell for the full duration, the spell lasts until it is dispelled.

The target's form is maintained by magic, so what is simulacrum duplicating? By strictest RAW, it duplicates the creature as it currently exists. The permanent-but-dispellable true polymorph wasn't cast on the duplicate, so the duplicate takes the previous form as soon as the simulacrum casting completes. I'm not sure how satisfying a solution that is, so a DM may want to rule differently, but it seems the most in-line with what's actually in print.