Phrael would signal for those that followed behind that the room was empty with an 'ok' hand gesture before moving to the end of the corridor and opening the next door. 

That door is also  unlocked, but this time you reach a corridor that extends to the north and south, One path ends in a square room, on the other end you can see a guard post (the other end of the arrow slits) and the corridor continues to a corner that turns to the east.

(Is there a guard? )

Quote from Eremis >>

(Is there a guard? )

 One goblin, sleeping near the arrow slitnfacing the circular hub.

Phrael would creep up to the guard, short sword drawn and stab them in the neck. 

Attack: 9 Damage: 9

You would have advantage since he's unconscious, and it would be an auto-crit since he is incapacitated at the moment. Same for Rin and Kebron if you guys coordinated the attack, though it would probably be overkill. 

That's right 

(So I roll a second d20 to try and hit or I just roll a 2nd damage die?)

Both (in case you hit the goblin, we can use the second damage dice for the crit)

Attack: 10 Damage: 4

Rin shadows Phrael, and if necessary he uses his shortsword to finish off the goblin. "There aren't many creatures I'd kill in this manner, but a goblin is one of them," thinks Rin to himself.

• Shortsword attack: 13 | Slashing damage, if successful: 7

Both of you failed at hitting the goblin (since the places is completely darks, Rin attacks without advantage) 

Phrael got distracted buenas Rin's footsteps behind him...

(Is the goblin still asleep?)

The goblin mutters something in weird tongue (unless you speak Goblin)

 And then he goes back to sleep. 

By the way, here is a map:

Incredibly impressed by the goblins luck, Phrael puts his shortsword away and instead works on tying the goblin up with the 50' of rope in his pack. More than enough to bound and gag, the trick however is not waking the goblin...

Sleight of hand??? 23

That was so dumb. Yet, so hilarious. XD I love it! And that last crit just made it perfect!

hahaha wow! yes, you managed to tie the goblin firmly without waking him up... I guess with piece of cloth in its mouth and all, right?

ok, so as you can see, there's still a corridor that goes down and east and the room back through the other route.

What next? 

"This is a tale no one will believe, Phrael," remarks Rin. "Let's hope we have better luck with our knot-tying." 

Rin holds his shortsword to the goblin's neck. In the off chance that the little monster wakes up, hopefully the steel blade will encourage it to keep silent. 

"That's been most our adventures. Stick around, Rin, it only gets better." Phrael replies as he finishes tying up the goblin then moves to the arrow slit, peeking through to find Flux. He's not sure what might be at the slit across the way, but at least this one has been taken care of. Pulling out some flint from his tinderbox, the wood elf would signal to Flux. No message set up, hopefully the intentions were clear. 

Flux perks up more from the noise than any light. The column in the middle made getting a proper angle difficult on things. Taking it as the signal nonetheless, Flux dims the Light of his holy symbol by adjusting his cloak over it, and begins making his way around the long way of the column. With a literal hope and a prayer, as his armor wasn't the quietest, to say the least.

Stealth: 2 plus Guidance: 3.

If he can arrive without incident, Flux takes one look at the bound and unconscious goblin, opens his mouth to say something, then closes it and gives the group a firm nod and a thumbs-up. He then backtracks to try and find something to bar the door from the other room. Unless his spear would suffice as a decent stopper?

Heavy armor disadvantage suuuuucks.