Ironmonger42 says:
As the doors close, nearly everyone makes it out before the doors slam shut, either through their own speed or the aid of others. The only one that doesn't is Isaiah, who does manage to get out with his magic but not quickly. The door slams shut as he is the only one in the room. With his magic, he makes a hole in the door, but not after poisonous gas comes out of the snake heads. He inhales some of the gas and then walks out of the room. The hole he made allows the gas to escape, but rather than dissipate it gets thicker and thicker. Everyone retreats back the way they came and are forced back as far as the bridge, where the heavy smoke fills the entire chamber before the room and flows down the rock face to the river below. The gas gets thicker and thicker, and there is no way of knowing for how long the gas will flow. Unless you are all willing to go back in or have some way past it, this option is now not available.
OOC:
No one described which way they ran, only that they moved, so i'm just going to say you all went back the way you came in.
Isaiah will take some poison damage, as seen below (
18 poison damage before any resistances), but after that it seems no one will get hurt but now you must figure out how to use the other path or get rid of the poison gas, as you ran from it rather than dealing with it.
Above is still true, but you went the left way instead of retreating the way you came in.
As you all make a break for it, Kelnes causes a wall of water to appear in the doorway with the door containing a large hole in it. As long as the wall persists, the gas does not escape, but you get the sense that the gas and building and building and there is no sense of it stopping. if the wall were to disappear the gas would not only flood your way but likely be so concentrated that it would kill you all in seconds.
At the moment, there appears to be a small room ahead, followed by a larger room, and just before the larger room the hallway veers right. the smaller room appears to be a storage room, but mostly containing food supplies, nothing of interest to you (no need to roll, just declaring no interest). The larger room seems more comfortable, and you see a magical mechanism on the wall. You might try to decipher it with knowledge of magic (DC 25 Arcana, max 2 people can roll or 1 with advantage if person with proficiency "helps" the other with proficiency, functionally same thing).