Prologue: arrival
It is your turn!
"Go ahead and get that door open while I take care of this zombie. There are more right behind us."
Eldrick casts a Firebolt at the zombie.
Rolls
Ranged spell attack at Zombie, damage - (d20+4, d10)
d20+4 : (15) + 4 = 19
d10 : (9) = 9
Meanwhile Thorem - you see less zombies as they are slower and disappear in darkness. You also don't see skeletons any more, but you hear all of them moving your way and you hear 2 arrows shot. in a while after that you see skeletons in the dim light of you light spell coming your way and preparing next arrows.
Next round of combat, Thorem - your turn.
Rolls
Skeleton 1 arrow to hit, advantage - (1d20+4, 1d20+4)
1d20+4 : (18) + 4 = 22
1d20+4 : (2) + 4 = 6
Skeleton 2 arrow to hit, advantage - (1d20+4, 1d20+4)
1d20+4 : (5) + 4 = 9
1d20+4 : (5) + 4 = 9
Skeleton 1 arrow damage - (1d6+2)
(4) + 2 = 6
How far away is Eldrick behind Thorem right now?
How far away is Thorem from the old large wooden double doors ahead of him.
How far away is the zombie from the cave that was firebolted by Eldrick for 9 HP of fire damage?
How far away is the 12 zombies chasing us?
How far away is the 2 skeletons that have been firing arrows?
How far away is the 2 skeletons that didn't fire arrows last round?
How far away is the zombies coming before that were out of Thorem's vision range that he heard? (Big guess.)
How far away is that cave that the zombie came out of that was firebolted now should Thorem even fathom to go there?
I think that is everything and if not than do add more!.
Thorem had no time to analyse battle situation and measure distance. He hear/understand that undeads are following but distance is increasing, because skeletons are busy firing while zombies are slow. Due to increased distance you don't even see as much of them as you saw before 6 seconds. The burning zombie is current at the same distance as the closes of other 12 zombies (he was not smart enough timely change running direction).
Two arrows shot from the dark are making your way and this time one aims each of you.
Rolls
To hit Eldrick - (1d20+4)
(20) + 4 = 24
To hit Thorem - (1d20+4)
(19) + 4 = 23
Critical damage Eldrick - (2d6+2)
(46) + 2 = 12
Damage Thorem - (1d6+2)
(6) + 2 = 8
What do you do?
Eldrick - you have to roll death saving throw, I'm afraid.
Rolls
Athletics DC(10) - (1d20+5)
(20) + 5 = 25
You see rails and an empty mine cart. Rails go slightly downhill into the dark cave. (I mean it looks like if you would push the cart some 50 feet it would continue on it's own because of relief. Size of the cart is enough for both of you to barely squeeze in.
The cart is 40 feet from you.
Rolls
Medicine DC(10) - (1d20-1)
(14) - 1 = 13
Rolls
Zombie str check - opening door - (1d20+1, 1d20+1)
1d20+1 : (1) + 1 = 2
1d20+1 : (4) + 1 = 5
Assuming that that really doesn't look to be at all promising, he is instead considering picking up Eldrick and into the cart we go. He of course would be spending that round pushing it. He doesn't expect Eldrick to come conscious any time soon. Thorem gets to do the Heroic Escape.
Rolls
Athletics Checks - (1d20+5, 1d20+5)
1d20+5 : (11) + 5 = 16
1d20+5 : (1) + 5 = 6
but before anything else happens Thorem manages to make cart moving on it's own, jump into the cart and hide from more arrows.
Zombies almost reach cart, but it increase it's speed and you are saved ... at lest from zombies.
Rolls
Skeleton 1 arrow to hit - (1d20+4)
(20) + 4 = 24
Skeleton 2 arrow to hit - (1d20+4)
(11) + 4 = 15
Arrow 1 crit damage - (2d6+2)
(26) + 2 = 10