The Manor
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"I figure you go first, Ealdwig second, and me last. We don't wait for each other to make it, just run at the same time in that order."
And to Ealdwig he says, "Don't worry about the unexplored rooms. I plan to come back and finish checking out the building. I just want that signet (or whatever) first. At the least, we have to retrieve our fallen compatriot's body."
The idea is that I figure the arrow is a warning that something will hit you from that direction (from the north). We entered the room (in order to check the sulfurous wall), so entering the room didn't trigger anything. Presumably, it is opening the (presumptive) exit door that is the trigger.
My plan, such as it is, is to stand in the little space outside Room 3. Then I remotely open the door. If 1) there is still a valid path to the exit door, 2) the open door seems to show the outside, and 3) death isn't continuously raining from the north wall, then we make a break for the exit door. The stated order is so that if running to the door triggers anything, Blornvid takes the first shot. If nothing hits him, then the safest slot is probably second, so Ealdwig goes. I go last so that if anything does happen delayed or if Blornvid falls into a pit or whatever, Skeeve can try to quickly magic up a solution. Probably not the greatest plan, but still, a plan.

Standing in the short hall between the two rooms (7 & 3) Skeeve tries to open the door but it is locked and must be unlocked before being opened. You search the door and Ealdwig is convinced it is trapped but the trap trigger is opening the door and does not see a way to disable the alarm on the door. After unlocking the door he retreats and Skeeve again tries to open the door.
As soon as the door is opened the entire western wall spouts gouts of flame and shoots them across the entire room, filling in and overflowing into the open doorway. The flames persists and after several seconds the door swings shut and the flames abate.
A little more sarcastically than expected Ealdwig says got any better ideas? Then realizing that was, maybe uncalled for, tilts his head and smiles a little, sorry. More upbeat, Ok, so what else can we try? There has to be a way to disable it, right? Anyone see an off switch? And Ealdwig gives a small giggle.
Thinking for a few minutes, Skeeve says, "Let's find out how bad the flames actually are ..."
He goes back to the previous room, retrieves some of the rotting crates and places them in the flame room. He has the other two stand back in the previous room, opens the door again, and retreats when the flames spit out. When done, he goes to check on the boxes.
The trigger is mechanical so there should be a way to disable it but to figure out how additional investigation is required. . . .
Also, that door seems to be surviving horrific fire pretty well? Does it detect as magic?
As for investigation, Ealdwig saw some kind of "wiring" attached to the door. Skeeve would like to trace it and see where it goes, if it isn't completely covered and hidden. (I am using Perception for this, as he is just trying to find it and see it, not figure out how it works.)
Rolls
Skeeve: Magic Scan - (2d6)
(45) = 9
Skeeve: Investigate "wiring" - (3d6)
(465) = 15
Ealdwig paces around Room 12, walking in circles. His small hands are moving around like he is putting a puzzle together or something. Speaking softly almost to himself, ok, so what do we know. I checked the door and it was locked. The door is trapped, we know that for sure. When you open the door flames, huge, hot flames shoot out and sunder the whole room and part of the little hallway. No way to get through there. We think there is a door that leads outside the building.
Ealdwig, making doubly sure there are no flames and the door is closed, goes back to investigate the door again.
Rolls
Check around the door - (2d6)
(32) = 5
Check for hidden mechanism - (2d6)
(43) = 7
Search for key in hall and Room 12 - (2d6)
(44) = 8
Trying to remember if I saw a key or something that could hold a key anywhere in the building - (2d6)
(32) = 5
Rolls
Skeeve: Check Flames for Magic - (2d6)
(32) = 5
Are the flames in room 7 or 3? I think 7 but want to be sure. And is it the door that opens into room 3 that triggers the flames, or is it the door from 7 to the little nook before room 3?
Blornvid is heat-adapted so he gets to roll at advantage for heat and fire saves. If we come up with a plan, he should be the one to give it a go.
The boxes are only involved because Skeeve intentionally took some out of room 7 and moved them into room 3 to make sure the flames were real.
Based on how the flames are being described, Skeeve is starting to think they are magical, at least to a certain degree. (Which is why he wants to find out.) The self-destructive part of Skeeve now wants to take the demonic idol, put it into room 3, then trigger the door again. It'll either completely destroy the idol, or animate it. I don't know which.
1) When we tried opening door A to this point, we were standing in the little one-box corridor between rooms 7 and 3 and Skeeve opened door A with TK. He's done that twice now. Once was the first time just to find out what would happen. The second time was with the boxes in the room (that are now ash). My understanding is that the entirety of room 3, and maybe even the little one-box corridor between rooms 7 and 3 are filled with flame. We were able to get out of the way quickly enough on the first attempt. Skeeve knew it would happen on the second and (if any) later attempts, so he can stay clear more easily.
2) Ealdwig and Skeeve are trying to figure out if there is an obvious way to disable the trap. Both characters have a few rolls each to try out different things and see what works. If we can, great. If not, we have problems.
3) Skeeve's last question is how fast the flames move. What he's thinking is that if the flames are not effectively instantaneous, then one person can go directly to door A, physically stand right next to it, as close as possible, open it, dive through it, and avoid almost all or all of the flames. Now there is no run across the room. It is just a literal open door and leap. We could even have Skeeve open the door remotely with TK so the person standing next to the door only has to worry about leaping. If it works, we might even have Blornvid open the door (from the outside) for Skeeve when it is his turn. I am assuming it doesn't work as the flame is instantaneous, but I do have to ask.
4) I am serious about the demon idol. Assuming we can move it, I wanna know what happens if we can put it in room 3. (I am not sure how big the idol is. Is it large like the idol from the old original edition AD&D 1e Player's Handbook cover? Is it small like the golden idol at the start of the original Indiana Jones movie? Somewhere in between? Is it built into the wall? Is it free standing? Honestly, assuming we can't get past the flames, that's where Skeeve wants to go next.)
The flames themselves are not magical though Skeeve suspects they may be crated or enhanced somehow by magic. There is a slight delay in opening the door and the arrival of the flames there so one person might be able to exit before being burned. If they are really quick?
Rolls
Check traps - (2d6)
(12) = 3
Pick lock - (2d6)
(26) = 8
Rolls
Skeeve: Help on the Cupboard - (2d6)
(45) = 9