Oreus will be the last to follow. He enters in the little room but peering outside to see if somebody reacted to the alarm.

Oreus doesn't see anyone rushing to the area. 

Shadow will continue to follow, if he can make it  to the corner he will peer around to see what awaits

Shadow dashed and uses acrobatics across the loose floor 15

Tyris dashes after the others.  

Acrobatics: 16

Shadow crouching over in the 5 feet tunne manages to maneuver through the tunnel and around Laladak only to crash in the skulk (who had dropped to all fours to crawl) at the corner. Even after crashing into him you have a hard time seeing him, but you do catch glimpses  of yhe wolf mask. 

Make a Dex save DC 12 to see if you fall down 

His DC is 10 as he is lower to the group. Skulk Dex DC save 5

(If I can see the skulk befre I've gone more than 30 feet I'll take a shot at it instead of dashing)

11 dex save

Tyris moves through the broken door and around the tunnel entrance.  You can kind of see the skulk, he will have three-quarters cover (from Laladak and Shadow) and you will have disadvantage (because he blends in with the tunnel walls and prone from attack purposes)

(My ranged attacks ignore cover and three-quarters cover due to the sharpshooter feat.  Doesn't stop the disadvatnage for prone)

Coming around the corner and seeing the skulk down the tunnel, Tyris snaps up his bow and launches an arrow at it.

Attack: 18 Damage: 11

Alben moves down the hallway and enters the tunnel, grabs a stone from the floor, casts light on it, and throws it in the direction of the skulk. 

(I am assuming i dont need to roll to hit as i am not necessarily trying to hit him, but might as well. I am not sure of damage of thrown stone, 1d4?)

Attack: 8

Both Tyris and Alben attacks miss.

Recap of the round:

Laladak charged after the creature which disappears down the tunnel. He stumbles before the corner where the skulk disappeared. Shadow running past Laladak crashes into the skulk which had camouflaged himself with the tunnel floor when it dropped down to all fours to scurry down the tunnel. Both the skulk and Shadow made their Dex saves. Tyris shoot at the skulk but missed. Alben cast a light spell on a rock and throw it down the tunnel, it lands between Laladak and Shadow. Oreus is bringing up the rear, keeping an eye out for people (or things) converging on the alarm sound. He sees nothing. 

The skulk feeling threatened decades to try to flee again using a loping half crawling/half running movement (think Gollum in the LotR movies) he manages to go (acrobatics check 15) goes 60 feet up to northern tunnel. He again tries to stealth (with disadvantage as he's running in different terrain) 17.

Shadow you can roll a perception with advantage to keep him in sight as a free action, as you have Darkvision and are right on top of him. Everyone else will need a standard action to try to see him. If you dash you need a athletics or acrobatics check of 12 if you have Darkvision or are within 30 feet of a good light source, DC 15 if not. 

20 perception

Shadow will continue the chase and dash to keep up

7 acrobatics 

Shadow dashes after the skulk but only makes it 45 feet (15 feet from the skulk). These tunnels were definitely not made for 6' creatures. 

Looking for actions from everyone except Shadow 

Laladak will move to the Skulk and try to Grapple it. athletics 22

"I said stay put we are here to help!" 

Laladak will try to catch the skulk, but as you were 5 feet away from the skulk you will not be able to catch him this round (I will use that athleticis for your dash instead of a gapple). Laladak rounds the corner in pursuit of the skulk, but after running north 40 feet he suddenly is plunged into darkness (as the light stone Alben threw is at the corner). You can stop your movement not or try to finish your run of 20 feet (at which point I will determine by roll how far you get).

On a D8

1 = you trip on a rock and fall prone, ending your movement. 

2 or 3 = you trip on a rock, but manage to stay on your feet, ending your movement. 

4 = in the dark you shift slightly to the right banging into the tunnel wall, ending your movement. 

5 = in the dark you shift slightly to the left banging into the tunnel wall, ending your movement. 

6 - 8 = you manage to stay on track for 5 feet (repeat until you stop moving)

Oreus follows the others closing the rear everytime as he goes

Tyris continues to follow, attempting to get close enough to fire another arrow at the skulk, otherwise dashing to keep up the chase and move as far as possible.

(If I'm able to attack:)

Attack: 18 Damage: 12