Can you cast Speak With Dead on dead undead?

Question

The description of the speak with dead spell says (emphasis mine):

You grant the semblance of life and intelligence to a corpse of your choice within range, allowing it to answer the questions you pose. The corpse must still have a mouth and can’t be undead. [...]

Until the spell ends, you can ask the corpse up to five questions. The corpse knows only what it knew in life, including the languages it knew. [...] This spell doesn’t return the creature’s soul to its body, only its animating spirit. Thus, the corpse can’t learn new information, doesn’t comprehend anything that has happened since it died, and can’t speculate about future events.

Let's say my party is tracking someone with knowledge they need, who unbeknownst to them was betrayed by the big bad, killed, and turned into a wight. Obviously, in wight form he can't be targeted by the spell, because he's undead.

Once they slay the wight (assuming there's enough of the corpse left to talk with), is the resulting corpse still undead, and therefore unable to be the target of the speak with dead spell? Or is he just an ordinary corpse at that point, and this a valid target for the spell? Are dead undead still considered undead, or are they just dead?

Answer 1

Yes, a dead undead creature can be the target of a Speak With Dead spell.

This unofficial tweet by rules designer Jeremy Crawford on May 9, 2015 strongly suggests that a non-animate corpse, regardless of its previous nature, is considered an object for targeting purposes:

A non-undead corpse isn't considered a creature. It's effectively an object.

..this was followed up on June 12, 2016 with the following tweet:

A corpse is an object. An unconscious creature is a creature

...This establishes that once any creature is dead it becomes an object. Helpfully, the core rules also define an "object":

For the purpose of these rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects.

Monsters are defined on page 4 of the Monster Manual, under "What Is a Monster?":

A monster is defined as any creature that can be interacted with and potentially fought and killed.

Monsters have a "monster type" (e.g. aberration, celestial, undead); objects do not. If an object cannot have the "undead" monster type, it cannot be considered undead.

Therefore, a dead creature of any sort is an object, and no longer a monster. As such, it can no longer have a monster type, and rules pertaining to monster type would no longer apply.

Here's the thread that lead me to this answer: Is a dead creature's body considered an "object"?

Answer 2

The spell's text says No: you can't speak to the undead

A plain English reading of the rules is more persuasive than unofficial tweets.

You grant the semblance of life and intelligence to a corpse of your choice within range, allowing it to answer the questions you pose. The corpse must still have a mouth and can’t be undead. The spell fails if the corpse was the target of this spell within the last 10 days.

Note that it does not say "can't be an undead creature."
You have a zombie's corpse.
A zombie is undead.
The corpse can't be undead for this spell to work.
It says so right on the tin.

There is another problem with your example. Zombies can't speak, even though they can understand the languages that they knew in life. But, if you use a vampire spawn and your BBEG was a vampire, they can speak the languages they knew in life but the spell still fails.

Why?

The corpse must still have a mouth and can’t be undead.

That's pretty unambiguous.

There is a related Q&A where the accepted answer is that Speak with Dead does not work on skeletons, but, as that looks also like an interpretation of some rules that overlap in a particular way, I'll not call it gospel. (Though I agree with the conclusion).

Rules as Fun

If you are the DM and you would prefer that any corpse, to include an undead's corpse, is affected by the spell then by all means do that. Make sure the players are aware that you find that to be a more satisfying application of this spell. (I don't see this as being a significant change vis a vis power of the spell-it already has sufficient limitations).

You can have some fun with this...
Cleric asks zombie corpse: "Where did the vampire go?" and the Zombie replies "Braaaains ... "