I am going to be running a dungeon for my players which has a unique "rule" that the characters and NPCs cannot kill each other while inside the area. Wounds sew themselves shut and limbs and severed heads reattach by the will of this dungeon. The dungeon is meant to trap the players and force them to explore and find a way to escape. While there are still ways for characters to die, they cannot kill one another or any of the NPCs. 

I am trying to brainstorm additional encounters or events that can take place inside this dungeon that make the most of the fact that the characters cannot die. Perhaps they encounter a powerful NPC who ties into the story there who would normally be able to kill the players in any other circumstance, but can't while they are trapped. Perhaps a room could be added or modified to make for a puzzle that requires players to exploit this non-death mechanic to solve it.

I want to hear what ideas come to everyone else's minds when a condition like this is presented. 

You could have a passage blocked by a stack of spinning blades, and the only way through is to literally hurl yourself at it and let it slice you like deli meat before reforming on the other side.

If a character is decapitated, how do they reform? Do both the head and body continue to function, even if separated at a distance? Could you give yourself a hand by cutting off an arm and using it like a grabbing tool with your other hand?

If the dungeon also suppressed magic, being Mr. Potato Head could come in handy.

Maybe you could trap them in the dungeon with a recurring villain. Should you attack another creature, you lose the limb that you used to attack, and only regeneration or a similar spell can replace it. However, there can be an creature or effect that they need to escape that doesn't deal damage. Perhaps slow petrification from a cursed statue, or a dangerous but slow spreading disease from a creature or object. Whatever it is, make it clear to the players that only way to save themselves is to escape the dungeon. Only allow spells like greater restoration to slow the condition, not stop it, if the dungeon allows magic at all.

Sounds ripe for a kind of Prometheus situation where someone is tied up in there and killed again and again and again, in a very painful way, but they can't die. Or not even killed, just hurt again and again. The PCs can save the person, who can then either help them find a way out and become a recurring ally for the party. Or it can turn out the person was quite evil and had it coming and now what have they released upon the world? Or they both help the party find a way out and are evil to make for more interesting choices going forward.

If you can't die, does that include from old age? If so, maybe some person just moved into the dungeon to cheat death, and there's probably a hundred different ways that could play out. Off the top of my head, a wizard seeking time to finish experiments without having to go to the trouble  of becoming a lich, or a parent who brought their terminally ill child there (and what kind of "life" does the child have now, locked in a dungeon for eternity), an old couple who are still deeply in love and don't want to watch each other die, a rich person who realized you can't take it with you, so he moved to the dungeon, with his treasure hoard.

And in any case, I'd bet there's some god of death or the underworld or what have you who really hates the place since it's robbing them of souls that should be theirs.

Do they knit back together after they die or do they start to regenerate after just being wounded. I think it would be more dramatic if they suffer wounds like normal and you tell them they get triple advantage on death saves.

The first thing that comes to my mind is: "This is going to need a LOT of prewriting."

Because here's the thing with storytelling and D&D: You're either focusing on the plot, the characters, the action, or two of the three. You'll rarely get a moment to focus on all three outside of the final battle with the BBEG. For example: I like combat sessions and rely on encounters for sessions when I don't have enough time to do a lot of dialogue and description pre-writing. When I can prep is when my players get to play with some NPCs. This is a dungeon where combat is actually pointless, which means you'll be focusing almost solely on characters and plot (after a couple unique moments with dungeon traps and exploring the "can't die" mechanics.) 

My advice is to do a lot of prep for this, and I mean a lot. Have plot hooks, have character dialogues, have character monologues, be ready to do a lot of on-the-fly back-and-forths between the players and NPCs. Make the effects of this weird limbo on the character's psyche the focus, and you'll get a lot out of this mechanic.

Problem is, if your players are the type that aren't too into that kind of RP and storytelling, you won't get too far with that. Nothing wrong with combat-oriented players, but when you put combat-oriented players in a room where combat is pointless or forbidden, they'll be desperate to get out, and the harder you make it for them to get out, the more they'll hate you for it. More RP-based players will love the heck out of this (I know I would), but you'll really need to be prepared with a lot of content to make the most of it.

That's just my idiotic take on it ;P

So after the first encounter, how do you stop the monsters from continuing to attack the PCs? After 3 rooms, you have a LOT of goblins in the way.

I suppose there is some clarification in order here. The "dungeon" I am using is a haunted tavern which traps those that enter it inside. The "rule" comes about as the tavern acts to stop those that its trapped from killing one another and effectively "leave" the tavern through death. I believe it is still possible for the players to die from interacting with rooms within the tavern, but they cannot kill each other or any of the other "guests." The guests also cannot die of old age as some have been trapped there for hundreds of years. 

I don't know whether it is a core feature of Curse of Strahd or a homebrew add-on, but there is a mechanic where players can technically die, but they always get revived with some "dark gift". Depending on how you die, and where you die, you might come back as a Frankenstein's monster with detachable limbs, or you might gain the ability to vomit stomach acid, etc.

Basically, the more you die, the more "corrupted" or influenced you become.

Maybe a haunted tavern would revive players, but also lower their mental stats in the process. Lowering their resistance to mind altering effects, and effectively forcing the players to "enjoy" themselves, rather than being able to consider escape. The oldest victims could have degenerated into blacked out club ravers, while one or two "survivors" have only remained sane as long as they have because they are too stubborn to have fun.

For an interesting twist, the haunted tavern might be willing to lower its guard with the more vulnerable "guests", so the only way to find the exit is either to get debilitatingly drunk, or to succumb to its mind-altering presence. However, if you succumb, you have already lost the ability to want to leave. Thus, in order to escape, the players either need to walk a razor's edge, or attempt to coerce the other residents to give them clues of where to go.

For bonus points, you could add this to your Soundscape every time the zombified "residents" start to close in on the group: LMFAO Everyday I'm Shuffling

Adding Everyday Im Shuffling to this encounter would not even surprise my party at this point XD