Help me build this trap

[Type2Demon]

I'm putting together a liches lair for a party of 10th level characters.

One trap that I'm planning is a hallway that is 100 ft long by 10 ft wide by 10 ft high.
Anyone entering the hallway can make a perception check to notice that the floor is strangely warm. There are grated vents on the ceiling.

Anyone reaching the 50ft mark in the hallway triggers a proximity trap.
The floor of the hallway splits apart,each half sliding towards a wall of the hallway.
Below the sliding floor is a 3 foot deep pool of lava that runs the length of the hall. Anyone falling in takes damage from exposure to lava. Anyone above the lava would still take some damage from the superheated air.
The "Disable point" for the trap is on the ceiling right at the trigger point. (so someone would have to levitate, fly or otherwise get to the ceiling to disarm it and maybe trigger it in the process.)

How would you stat this trap out?
Would you give the party enough time to run out of the trap?
Would you do it differently? Would you add something?
Do you see any glaring flaws in the trap?

Opinions and suggestions are welcome.

[Phneri]

First glaring flaw is you're using a relatively easy to bypass trap and upping the lethality of said trap to compromise it. By 7th 8th level, let alone 10th, if the party runs into a room with even a remotely suspicious floor they fly through it.

A good trap is going to be mechanically devious but not instantly murder a PC. Not because this is realistic, because it creates appropriate tension. If you create a devious trap that immediately drops party members into 20d6 damage, they're done. if it's just exposure (2d6) the damage is relatively trivial and can be bypassed quickly. If it's easy to spot and bypass you're bummed cuz you spent 30 minutes statting a trap that never went off.

Now, the classic pit trap can be nasty, but it has to be almost undetectable to work, and it shouldn't either instagack or do minor damage to your party.

I'd use a number of glyphs of warding, triggered upon various events, with specifically-phrased suggestions. This, coupled with illusions, can make the pool of lava seem like a swimming pool, and in all this heat, wouldn't a swim cool you off?

You could also use a number of "quick! danger! step to your left!" wards to move PCs off the beaten path into the dead drop of the pit.

Maybe instead of lava use boiling tar? This offers more potential clues (odor), more danger (toxic fumes = fort saves) and less overall damage (maybe 10d6 damage for immersion?).

Also think of a rationale for the trap. Was was the purpose of this? Is it a creation of your BBEG or left by a previous occupant? How do the bad guys adapt to it?

Here's something I used in the past for example:

Pit Trap:

For a 7th-8th level party. They entered the lair of a cleric BBEG who'd had WEEKS to prepare his lair. His deathtrap was a 5' wide bridge over a 100' by 40' room with a 40' pit beneath. He'd used stoneshape over a period of weeks to smooth the stone (unclimbable) and create the bridge. He'd also placed several glyphs of warding and used an artist minion to paint several large canvases as stone floor and spread them across the pit, creating the illusion of a solid, cohesive floor.

15' and 25' in the bridge had glyphs of warding with the "step to your left" suggestion trick in place, which would plummet a PC off the edge into the pit. The middle glyph triggered a stoneshape which removed that 5' block and collapsed the entire bridge into the pit.

In the pit the cleric had trapped a black pudding, which attacked anything that came near it. At the other end of the bridge he'd placed two (rotating shifts, 6 men total) sentries (5th level fighters) with composite bows and greatswords. They were in place to engage melee at the choke or harass people in the pit/in the air with arrows.

When I used this I took out a druid pet and maimed 1/3 of the party, which gave my BBEG plenty of time to prepare his last stand (read buff himself beyond all belief).

[,]

Another, simple and inconveniencing idea is the 'Reverse gravity' trap with the pit in the ceiling.

The initial DC to spot should be high (How many people ever look 'up'?) and getting the character out of the roof might present a few problems.

Of course, if the party simply 'dispels' said reverse gravity? Then the ten X ten block of stone that was the 'bottom' of said pit in the ceiling also falls down. Which, in a narrow-ish corridor, will cause other sorts of problems.

An old idea, but used correctly a goody. :)

Best of luck with harassing your PC's. =)

[Type2Demon]

Phneri wrote:

First glaring flaw is you're using a relatively easy to bypass trap and upping the lethality of said trap to compromise it. By 7th 8th level, let alone 10th, if the party runs into a room with even a remotely suspicious floor they fly through it.

(edited

Now, the classic pit trap can be nasty, but it has to be almost undetectable to work, and it shouldn't either instagack or do minor damage to your party.

(edited)

Maybe instead of lava use boiling tar? This offers more potential clues (odor), more danger (toxic fumes = fort saves) and less overall damage (maybe 10d6 damage for immersion?).

Also think of a rationale for the trap. Was was the purpose of this? Is it a creation of your BBEG or left by a previous occupant? How do the bad guys adapt to it?

Here's something I used in the past for example:

** spoiler omitted **...

I like the idea of using boiling tar. I will be drafting that into trap.

After doing a review of lava in the RAW, I think it would be overkill. I'm not trying for a TPK, but this will be the first major obstical that the party will face and its how the lich will be gauging the party and their reactions.
It's also to put a little fear into the players to let them know that the place will be no pushover.
This BBEG likes to prey on adventures and soften them up with traps and encounters before finally finishing them.