OK, so the goal here is to present the party with a trap that will make them feel the ticking clock toward impending doom-- think the Spike Room from Temple of Doom-- without ACTUALLY killing any of the characters outright (I love my players and their characters are so much fun)

SOME THINGS TO KNOW:

The monk and the rogues have high to ultra high dex so saves and disarming the thing shouldn't be an issue for them but I want it to be challenging hence the STR and INT checks prior to disarming. Unfortunately, the cleric and warlock have pretty disappointing DEX saves and NOBODY has athletics except for the npc paladin. I'm considering making anyone who fails the DEX saves start rolling death saves until they succeed (manage to regain their feet), someone gets to them to stabilize them (help them up), or they fail (get sucked into the grinder... probably anterior chain first... ew)

SUGGESTIONS WELCOME!!!

Hallway Grinder Trap. This hallway is 10’ high, 5’ wide, and 55’ long. The floor and ceiling are stained with three-hundred-year-old blood, making it difficult to see the patterned floor. 35’ into the hallway an unseen magical gateway will trigger two heavy, iron portcullises to descend at either end of the hallway and portions of the floor to retract— revealing the floor to actually be a series of gears designed to grind whoever is in the hallway into gore.

It will take three rounds for the gears to reach full speed. During those rounds, the hallway will become difficult terrain, requiring a successful Dexterity save to move across safely.

Round 1: DC10 Round 2: DC14 Round 3: DC18

Any character that fails a saving throw will be dealt 20d10 crushing damage. There are wall sconces every 10’ at staggered intervals and mounted 5.5’ from the dungeon floor. One character can cling to each sconce while three characters can cling to any single portcullis. The release lever is located on the wall just beyond one of the portcullises, but it is rusted in place after 300 years without use. Beside it is a panel, which can be pried open (DC:16) to reveal the operating mechanism. Intelligence Check (DC:13) to determine the course of action to disarm and a Dexterity Check (DC:16) to disable the device.

 Any character clinging to a sconce will require an Athletics Check every round. The first check will be at a DC:10, with each successive round raising the check by two (12, 14, 16 and so on…) The portcullises can be raised once the trap is disarmed.      

What level is the party? Because 20d10 damage can possibly kill them outright due to Instant Death rules.  Even if one survives the initial damage and start making Death saving throws, it will automatically fails further Dexterity saving throws if unconscious and upon taking damage will suffer a death saving throw failure, that is if the damage doesn't equals or exceeds HP maximum and suffer instant death as well.

The Dexterity saving throw DC increase is steep and depending on their level, DC 18 could be very hard to succeed. Is there damage on a success?

Anyone have access to Teleport, Misty Step, Dimension Door or other teleportation movement could easily bypass it, so is anyone who could polymorph into something small enought to pass through the portcullies bars. A way to further limit some movement or teleporation could be to make portcullies all solid wall be instead for total cover that block vision to prevent movement and effect requiring to see target point or destination.

So, if you need a trap which looks deadly but isn't...try a sliding scale of damage/DC approach.

1): When you unveil the trap be very serious and tell the party this could be life ending and they need to think how they want to proceed and make sure they are aware the area is now difficult terrain.

2): Let the party know they can help each other. If they want to use the Help action then they can give someone advantage on the save. You might also say the chracter using the help action then has disadvantage on their save as they are not completely focused on themselves but that depends on you and your players. But I would also say if you are recieving the Help action you cannot then use the Help action to help someone else, so if person A Helps person B, then person B cannot use the help action to Help person C.

3): All Saves start off at DC10 or whatever you think is a good challenge for the party overall.

4): If the saves are made, great nothing bad happens. If the save fails then they take 1d6 damage per 2 points (rounded down) they failed by. So if it was DC10 and they rolled a nat 1 then they take 4d6 damage BUT the DC for the next save goes up by a like amount..so the next DC10 save just became a DC14 save. This can then be explained as the gears snagging clothing, twisting ankles as they skip from cog to cog etc. I'd probably keep this to d6's just because the average damage per dice is 3 vs 5 for a D10 (I might be wrong on that) and as the party is made up of a rogue, monk, cleric and warlock you probably won't whittle their HP down too quickly barring some massive blunders on their part.

5): I'd probably leave out the Athletic checks and instead have a stone slightly recessed in the wall near each portcullis with a DC15 or so Perception or Investigation check to find them, if both are pushed in the trap deactivates but they require constant pressure to hold them in or the trap starts again. Given the 5ft width of the corridor a two handed weapon would be long enough to wedge into place and hold one stone in place thus enabling everyone to get out.

6): If you wan them to get out alive, be prepared to fudge damage rolls. Letting them get out with their HP in double digits can be just as dramatic as letting them get out with 1.

I think that all makes sense.......

This is great. Appreciate the response and the Help action is a great idea. 

Also like the damage suggestion actualized as a twisted ankle, etc... the real problem I was facing with partial damage was the physics of the thing; this grinder is gonna pull in an chew up ANYTHING it gets ahold of, but the damage as incurred WHILE succeeding at avoiding the mechanism is a great idea.

I did have a rethink on the athletics skill checks...give the players 2 obvious ways of getting along the corridor, option 1) Dex saves to bounce along the cogs/gear/grinders or option 2) have hand holds/ladder rungs in the ceiling that they can swing along like you do with monkey bars on an assault course. The players just need to roll a few Strength/Athletic checks but if they fail they fall onto the grinders and then have to make Dex saves to get out.

There is also option 3 which is any creative ways the players can think of getting out such as using Magic Missle, or even better Disintergrate, to blow the bars off the portcullis and you might also want a super secret passage the builders put in to escape the carnage in the event they found themselves setting the trap off; have a secret door with a DC in the 15-20 range to locate it and it leads to a safe room then runs parallel to the trap.

That's a super interesting idea.

For context, this trap was built in the hallway between the lower portions of an ancient arcane laboratory where all manner of nasty experimentation was done and the upper levels where artifacts and, eventually, the exit is found. So this trap is, in part, PRECISELY why nothing has managed to escape in 300 years, right? The characters only entered the dungeon by accident, stone shaping an escape from an adjacent prison cell in a different subterranean structure.

Thanks again for the ideas!