Tyris shrugs.  "We've left a ton of passageways undiscovered.  I vote we see where the tunnel goes first."

Oreus :

"I agree but , just out of curiosity, in order not o leave anything , let's just open also the door with the "I" rune in the middle of the main hall, it will just one minute, then we can continue with Tyris proposal. So far we didn't understand the core of this...building...dungeon...call it as you wish: who built it? why? what was that monster guarding? There must be a connection with the strange events happening in the city..."

The Inn is closed for the night (it's around 2am). So Shadow offers you all to have a midnight snack at his house (assuming it's ok with Shadow). You spend the first 20 minutes discussing what you have seen, while eating a hearty meal. Then you turn to discussion your next move. Oreus wants to check out the I door in the main hallway, Tyris is in favour of exploring the tunnels Laladak thinks you should go back to the room with the broken glass. (So far Alben has been concentrating on his food and hasn't given an option).

Shadow as the elected leader (at least for now), the final say is yours. 

(We didn’t leave we just stayed in the room Alben and I first woke up in we didn’t leave the dungeon). 

Well I am hesistant to leave an enemy at our back, Tyris is right we should continue with the tunnels for now and try to find where that creature went.  We will go back and retrace our steps east and the turn south and continue down the tunnels for the moment, just everyone he alert

During the break Oreus searches inside his trunk to recover his biscuits, just to realize that during the fall in the middle of the battle with the monster, they got all splattered here and there.

Swearing inside his mouth, he eats the few pieces that are not destroyed and passes the rest of the short rest cleaning the pouch.

Finally he nods at his friend Shadow, gets back on his feet and he is ready to go.

After sharing a meal, Shadow leads the party back through the tunnel they came through and at the intersection turn south hoping to find something interesting. 50 feet south of the intersection you come across a short tunnel heading west with bright light coming through what looks like brambles. The southern tunnel continues another 30 feet and opens into a room.

Alben inspects the light and brambles for any evidence of magic (brambles grown as a wall or warning system) arcana: 4

Alben's instinct tells him that magic is involved, but searching through his lore provides no answers. 

Laladak would like to help Alben and talk about his experiences clearing trails in the magical forest nearby Beeftown. 

Using Beeftown vegetation as a reference( :P ), Alben reaches back in his memory to try and remember anything about the brambles:22

There's not really enough visible evidence (If you decide to go into the room I will use the 22 arcana) to determine what really is going on. But differently there is magic at work here. You don't believe its harmful. 

I relay the information to Shadow. "I think we should head this direction"

Alright let’s go investigate further.  Shadow will lead them toward the magical brambles

Shadow leads the party through the bramles and enters a magical place. You see...

This section of the dungeon looks like a small forest. Four great trees sprout from the grassy floor, their leafy crowns fanning out twenty feet overhead to form a roof of deciduous leaves through which rays of sunshine filter. Brambles and thickets form impassible walls, although bramble archways set into the north and south walls lead to dark passages and chambers beyond. There is a gear door with a D glyph on the southern part of the western wall. A gentle zephyr causes leaves to rustle, and birds chirp merrily in the boughs above.

Alben looking around knows that this is a spell similar to Hallucinatory terrain (not sure if there is a 5e version of this spell) and that there is probably are really stone pillars where the trees are and so forth.

Wel this is unusual.  Investigate the area 7

Does anyone help Shadow Investigate or investigate on their own?

Laladak stands Ready to strike anything hostile. 

Alben will help with the investigation: 14

Shadow Investigates the trees and brambles but doesn't find anything interesting. However he does look through the southern archway and sees the remains of a ruined dinning hall with four gear doors, 2 to the east and 2 to the west. 

Alben Investigates the northern part of the forest and something about the northwest corner catches his eye. Give me a 2nd Investigation Check with a +2 bonus (+5 total).

"Huh.  Didn't expect to find a forest down here," Tyris says.