Rizo grunts from the impact of a larger rock just after he finished his spell. "Let's hurry! Please! "

Minty runs to the mouth of the cave (Move Action, Bonus Action).  "If you want me to try and open it, it would make sense for the rest of you to clear the area and I will run back and give it my best shot.  Rizo, if you are OK, you may want to stand by with the water, since it looks like an acid trap to me."

 42, Damage: 0

Rizo, looks at Minty with a confused look. "But I only spread the water on the floor. You think that could have adverse interactions with the trap?" 

When Minty arrives to the mouth of the cave, he sees something glittering in the sunlight around the dragons neck. Closer inspection reveals that the Dragon wears a necklace with a crudely forged key. If that key fits in one of the two key holes of the chest it might make it easier to unlock it and, potentially, even to disarm the trap. (Technically it will be lowering the DC by 5 for both, unlocking and disarming, leaving it at DC 10 both.)

(Egan and Justin didn't retract their action and Minty went out of the cave, so I assume the chest moves to K-02, and will keep moving until I hear or see otherwise - I'll give it a day per turn. With this we go into the fourth round. - Ceiling collapse [%roll]1d100[/roll], Damage: [roll](if([roll:-1], <5, 1d6, 0))[/roll%])

(Oh, I thought Lilgrea was going to freeze it over, so I waited with posting. Can someon do my rolls? The editing is bit difficult on mobile)

Ceiling collapse for Justin in round 3: 30, Damage: 0

(Short summary: I think only Rizo has that spell and we will just assume that the way is 15 feet shorter, that means at turn 10 the chest would leave the cave if we continue that route, we're now in turn 4. Lilgrea can use mage hand to clean up the debris in the chest's path, though, to make sure that there is nothing slowing down the process. Minty just found one of two keys, too, which makes it easier to try his solution.)

(Lilgrea has no spells to freeze anything meaningful - all she's ever managed to freeze is the top of her beer when she uses Prestidigitation to try to cool her beer a bit)

As suggested, Lilgrea will use Mage Hand or Prestidigitation to clear the path in front of the chest to try to speed things up.

I think she's now in round 4 and probably due a ceiling collapse check: 64, Damage: 0)

Egan, who also has Mage Hand, assists Lilgrea in clearing debris as best he can as he helps lug the chest.

80  Damage so far is 0.

Justin keeps pushing and uses dash to push more, taking exhaustion.

With this we only need a DC 5 Athletics check for Justin and Egan who roll with the better of their skill with advantage for helping each other. If successful (it will be as Justin has +4 mod), the chest will move to K-04.

Ceiling collapse for Justin round 4 84, Damage: 0

Lilgrea can, from here on, help from the outside of the cave so she doesn't need to worry about the ceiling any longer. 

Rizo freezes the water on K-05, on which the chest will slide easily so that it will count as free movement. 

Ceiling collapse 64, Damage: 0

The next dash-action-push needs to pass a DC 10 Athletics check (with advantage and Justin's +4 mod). 

We're now in round 5 (and use: Ceiling collapse [%roll]1d100[/roll], Damage: [roll](if([roll:-1], <6, 1d6, 0))[/roll%])

Rizo pulls water on K-08, and leaves the cave, too, but ready to substitute Justin or Egan should they show signs of exhaustion or get injured. Ceiling collapse 96, Damage: 0

Summary (round 5):

Chest on K-04, K-05 frozen for free chest movement, Justin and Egan inside the cave, both pushing the chest out. Lilgrea and Egan clean the path, Rizo prepares K-08 for skating and Minty prepares to unlock and disarm the chest once in safety. 

It looks to me the chosen path is to push out the chest, so we can fast forward and just roll if there is any damage or exhaustion, if you want to speed things up (check below). 

Justin continues the chosen path.

99, Damage: 0

Atheletics: 6

Forgot advantage: 24

Justin and Egan strain to scrape the chest over the cave floor as fast as possible, sweat dropping from their foreheads. Legs, arms, the whole body burning from the effort. Rocks raining around them to urge them on. After half the way it becomes clear that exhaustion has set in as they push forward at the utmost limit of their abilities. 

For Egan in round 5: 29, Damage: 0

(Justin and Egan are at exhaustion level 1. Their next Athletics check will be without advantage and at DC 15.)

With this we're in round 6 (and use [%roll]1d100[/roll], Damage: [roll](if([roll:-1], <7, 1d6, 0))[/roll%])

Rizo looks to Lilgrea and Minty saying, "They've almost made it! But we have to help them, I mean ... we should take over, they are at their breaking point." 

Egan's Athletics check:  14

Justin and Egan labour to move the chest forward, which is now very near the cave exit (K-10). They push their bodies beyond their limits, obviously suffering through the ordeal (Level 2 of exhaustion - half movement speed, next Athletics DC is 20, unless someone fresh pushes). 

Justin’s and Egan’s check, respectively:

6, Damage: 3
17, Damage: 0

We go into round 7: 
[%roll]1d100[/roll], Damage: [roll](if([roll:-1], <8, 1d6, 0))[/roll%]

Rizo can’t watch any longer and enters the cave again, asking Minty and Lilgrea if one of them can join him, “They need help we need to take over, can one of you help me to do the last push?” 

41, Damage: 0

(If they let us take over the last bit and one of you helps Rizo, you can also roll Athletics DC 5 with advantage and mod +1 along with the ceiling roll. After that the chest will be safe and we can see what is inside!)

Minty rushes in to help Rizo with the others.

Athletics:20

11, Damage: 0

Minty and Rizo rush to help Justin and Egan in their last effort to push the chest out of the crumbling cave. With parts of the ceiling collapsing all around them, they manage to shove it out of the cave mouth. Justin and Egan stumble out of the cave while feeling how their senses fail them. First, shouts around them were reduced to mere mumbles, now a strangling blackness encroaches their vision. They can't help but sit down and lean against the chest to avoid toppling over unconsciously. After a few moments they manage to catch their breath and slowly regain their vision and hearing capacity, albeit both will be severely weakened until they can rest truly for an extended period of time (Justin and Egan have two levels of exhaustion, one level can be recovered during one long rest, for the rest of the adventure, you will move at half speed and have disadvantage on ability checks). 

Everyone in the party is eager to open the chest to see if all this effort was worth the cost, or if Cerdis will have the last laugh after all. 

A few minutes of rest is enough for Justin and Egan to be relocated to a safer distance, so that Minty can unlock the chest and disarm the trap. Without the pressure from raining rocks and, additionally, with the key he found around the Dragon's neck it is a relatively simple procedure that he can complete with confidence. Everyone gathers around the chest when Minty finally pushes the lid open and the party finds...

For XP, I think the environmental hazard after Cerdis was a hard challenge (we've got 4 levels of Exhaustion from it which is substantial) and, additionally, the chest was locked and armed with a deadly trap. A summary of all the experience has been kept up to date in the first post of this thread.

There are two questions that remain:

1) Do you still want to do the extra content that I prepared some time ago for finishing this adventure? There might be two combats, one of them totally optional, but either can be tough, especially with exhausted party members. There's XP to be gained, sure, and zero to two treasure rolls but they don't come free. If you prefer we just get back to town unbothered.

2) There is a best role-play award at the end of the adventure worth an additional 250XP. Personally, I had a blast with all of you and I would propose not to vote but instead to distribute it equally - 50XP for each.

Please, voice your opinions. 

Reward: 16

1) I'll follow the majority

2) agreed!

1- It's certainly tempting to go for the extra content, it's all been a lot of fun! I'll understand though if people would rather get back to town.

2- Sounds good to me

3- Ooh - treasure, exciting! 17