GreyWord says:
The slab reduces and not fitting into it's place any more it falls
OOC:
OK time's up, "door" enlarge back. Please not quotation marks I use when call the slab a door, because these are no mechanically doors with hinges or something. Both "doors" where just big stone pieces and the first ones magically ascended into the ceilings.
The slab enlarges until it hits the side walls. You can guess it was a little bit bigger nut the magic can't break the walls. So the slab is currently 10x10 feet and stucked between walls, still standing almost vertically. It's up can actually be used as a step allowing anyone who climb up to rest (advantage on the roll to climb up this way).
Meanwhile, encouraged my how easy other made their way down Ixion start descending, putting his foot into the same tiny gap in the wall everyone else were using, but it just so happens that the gap cracks bigger and the food falls. Ixion still try to hold his wight with his arms, but weather is hot and humid in Chult (continent you are on right now) and Ixion's palms are all wet and slippery. So are the rope after few climbers.
OOC:
Either Throgin or Thaddeus can make a DC 13 STR saving throw if you try to catch Ixion (on failure you will both fall on the spikes). Your modifiers are 8 and 9 accordingly, so it doesn't much differ.
Going forward: if Throgin want to pull on the rope tied to any individual, Thaddeus can roll Athletics instead of a person he is pulling. More over if they work together with Thaddeus the roll will rolled with advantage. Obviously - they need to get up first