The difficulty comes from only being able to ask one question. It's very easy to figure out the liar, but it's much more difficult to figure out the liar and the correct door in the same question

That assumes the other guy holds to his principles in the face of death. If I were the dm, the act of tearing the other guy's head off and then threatening to do the same to the other one unless granted another question would at least grant advantage on an intimidation check

Because there are two doors and only one question. If you ask a known question unrelated to the door you find out who the liar is but lose your opportunity to ask them which is the correct door.

Then you still don't know which door is the correct one, you've just learned which guard tells the truth and you've used up your one question. The trick is to ask which door the other guard would tell you is the correct one and then go through the other door. If you've asked the lying guard, they'll lie about what the honest one would say and point you towards the wrong door. If you asked the honest one, they'll truthfully tell you what the lying guard would say and _also_ point you towards the wrong door

That's why this is a brilliantly played barbarian. They think they are clever but will still have to do things the hard way.

Is there an actual plot to Mimi, or is she just a complete chaos goblin?

I've always seen it as outside of their control. It's not that the lying guard _chooses_ to lie, it's that they're incapable of _not_ lying.

The first time I encountered a version of this riddle it actually wasn't Labyrinth. It was an old black and white episode of Dr Who aired on PBS when I was a little kid. Same scenario but if I recall, robots instead of guardsmen. I think the good doctor solved the riddle in the typical way of asking one robot what the other would say. I'm looking for it now but I can't find the scene.

One question per group. Also now you die because murder.

Simply goblin

I got an unexpected laugh from Rick and Mortys take on this. His answer was "you ever fuck this guys wife?" And watched them fight to the death.

https://www.giantitp.com/comics/oots0327.html

Knowing who lies and who tells the truth doesn't tell you which door leads to the prize and which to death.

That's why it's funny.

So the traditional answer here is to ask them to point at the door the other guard will say is safe. However, I'm curious, does anyone know of any other valid solutions?

This still doesn't accomplish the goal of knowing which door will kill you. All you've done is determine which guard is the liar.

So you ask them which way leads to the castle and you don't pick the way they say. If we're assuming that these things are actually bound by some kind of rule stating they literally cannot lie or literally cannot tell the truth.

But you asked your one question to the one remaining guard. You gained no intel.

I believe that's the joke. The barbarians intelligence isn't usually very high.

But the other party members haven't