Fire Puzzle Room

A puzzle room that qualifies as a Special when stocking a dungeon.

Picture this, but the bowl is ten feet in diameter and the fire is HOT.

The PCs enter a large rectangular chamber from the only open door.  The air in the room is noticeably warmer than the hall, the heat coming from the large black iron brazier near the middle.  As someone nears the center of the room, the brazier flares up, illuminating the rest of room and casting eerie shadows on the walls.  At this point, a doorway is visible on the far wall, opposite the entrance.  A large block of stone seals the door. Also visible in the light is a plaque which reads:

A fiery sacrifice 

opens the way 

to the closed room.

Other methods

open the way

to assured doom.

The first stanza makes it plain that casting things into the fire (which is a one-way portal to the Elemental Plane of Fire) opens the door.

10,000 gp worth of nonmagical valuables is the simplest method, with the door rising up slowly until the full amount is burned.  Brave PCs might try to slip under a partially raised door...

A half-dozen single use magic items (potions, scrolls, charms) or two permanent magic items work, again raising slowly.

Note that any DM-approved combination of the above work together.

The cultists who made use of this fiery lock saved money by casting living beings into the flames.

The door raises completely with a sentient being's life, or multiple nonsentients (animal companions, familiars, etc), voluntary or not.  So, in theory, a noble PC might throw herself into the flames to open the door.  In this case, just like the other unwilling sacrifices, they are irrevocably gone, consumed by the flames of the Elemental Plane of Fire, as it is the consumption of lifeforce that opens the door.

Sacrificing another being is an Evil act.  Alignment matters in my game, and PC alignment shifts straight to Evil.  Sacrificing different types of beings result in more than just an open door, however.

A cleric or champion of same or opposing deity - The PC is forever marked as an enemy of the faith, and the faithful sense and know it on sight.  1:6 chance a representative of the deity quests to destroy the sacrificer. The one doing the casting suffers nightmares for the next 1d6+1 months, with a 4:6 chance of having a long rest disrupted by them.  After that time frame, it becomes a 1:6 chance.

A cleric or champion of a Good deity - As above, but if the cleric is of the same or higher level as the sacrificer, the PC casting them into the flame gains a permanent level and the enmity of that deity and its minions.  If the sacrificed was level 9 or higher, the chance that something begins actively hunting the sacrificer becomes 4:6.

A sentient being - the being weeps, wails, begs, bargains, and voids its bladder and bowls while being cast into the flames.  The one doing the casting suffers nightmares for the next 1d6+1 months, with a 4:6 chance of having a long rest disrupted by them.  After that time frame, it becomes a 1:6 chance.

A Good sentient being - as above, but the sacrificer gains a permanent +1 bonus to a randomly determined stat for each Good sentient cast into the flame.

In each of the above cases, a fire demon (be it demon, efreet, or fire elemental) appears to the PC in a public setting - with witnesses - to congratulate the PC on joining it in its crusade against Good, then begins lashing out at random while calling to the PC to join it on a rampage.  It will disappear before it can be gravely injured.  

A magical creature -  as sentient beings, but it is the Fey that are angered and come gunning for the sacrificer.  Nightmares are the same.

A Good magical creature - as magical creatures, but sacrificer gains an immunity or power based on the specific creature sacrificed.  Nightmares are the same.

Animals - the least of the evils.  The animals whimper and scream and piss everywhere as they are picked up and cast kicking into the fire.  Whenever a long rest is attempted, 1:6 means nightmares and night sweats disrupt the long rest.

PCs that stand by and let others cast living beings into the flame are culpable and become tainted, requiring atonement and perhaps a quest before falling back into the good graces of their deity (and themselves).  They too, suffer nightmares, losing the benefits of a long rest 1:6 each long rest.

The second stanza suggests using other methods (typically knock or stone shape spells, but also brute force and anything else that doesn't involve a sacrifice) would be a poor decision.  

And it is.  

Dao, Monstrous Compendium, I think

Attempts to open the door trigger an ancient warding magic that summons an angry Dao (who is pissed because he has spent the last five centuries setting something up and the PCs have ruined it) and a full dozen earth elementals under the Dao's control.  The earth elementals come out of the walls and up from the floor, surrounding the PCs and begin melee, while the Dao directs things from afar.  One bit of direction it is partial to is having elementals try and knock PCs into the flame and immediate burnination.

Note that knock and stone shape and the other methods do, in fact, open the far door as surely as if something valuable was burned.  It is that using such methods triggers the guardians.  Defeating these guardians means looting them.  While the earth elementals merely crumble into rubble (that may contain uncut gems at the DM's discretion), the Dao has more entertaining options.

Source not me.  I would love to know, though, as I bet they have done other critters this way.

So either way, the door is opened, and the PCs pay for it in some way, shape, or form.  If particularly wicked (or unthinking) PCs end up calling down an extraplanar crusade upon themselves to save a few gp, well, that seems a fitting response, and certainly beats some alternatives.