I was reminded late last night that you made a path through the broken glass from where the party is now to the tunnel entrance where he is. Because of this I will not be enforcing the movement penalty I described above (except for maybe Alben, who is away from the path). I am going to assume you use the path to move into close range and then use your careful movement at the end of your movement to get into position.
Alben caught unprepared by the dark staker, retaliates with his strongest spell scorching ray hitting the enemy twice.
Laladak moves to in front of the mostly transparent creature and swings his greatsword, but the stalker is able to dodge nimbly out of the way.
Tyris casts his hunters mark on the stalker and shoots an arrow at him, hitting him in the left leg.
Shadow is up
Oreus your initiative is lower than the skulk's, who is going to try to escape the leash. I'm going to treat it like a grapple, give me a a Strength (Athletics) check. Both checks should be made with disadvantage (yours because of the would you just got and the skulk's because he is tied up), but since they cancel each other out just make a normal roll.
Shadow moves down to attack the Stalker with his rapier and makes a small scratch.
Fario use his cunning action to dash into position to attack the stalker with his Longsword Attack: 6 Damage: 11
Fellian moves to where Fario was and shoots the Stalker with his Shortbow Attack: 11 Damage: 10
Both Fario and Fellian attacks missed.
The skulk tries to escape his leash Acrobatics 23, but Oreus is able to keep control of the rope and hauls him to the back of the party.
(I deleted the first post because I forgot about the halfelves and did the skulk first prompting the manipulated banner both the halfelves misses the first time also, so by making a new post I actually gave them a better chance)
The Dark Stalker attacks Shadow (The only one in melee range who hit him) with his Shortsword Attack: 15 Damage: 6 + 7 Posion damage. Shadow drops to the ground unconscious, he is stable but poisoned for 1 hour, even after regaining hit points, and is paralyzed while poisoned in this way.
Laladak attacks the transparent enemy with his greatsword, but misses.
Tyris attempts to shoot the stalker but (1=your bowstring breaks, 2=you have a chance to hit Fario, 3=you have a chance to hit Laladak, 4-10=the arrow smashes into the wall) 9 the attack smashes into the wall above the stalker, shattering into pieces.
Fario attacks with both his Longsword Attack: 23 Damage: 11 and his Shortsword Attack: 18 Damage: 7. His two weapons both find flesh.
Fellian shoots another arrow at the enemy Shortbow Attack: 11 Damage: 11, but misses his target.
Suddenly another creature (it looks like another Dark Creeper, like you fought in kitchen) comes hurling out of the tunnel and attacks Fario with a Dagger Attack: 21 Damage: 12, dropping Fario.
The skulk manages to rip the rope out of Oreus hands and flees down the hallway.
Oreus chases him and drawing out his glaive manages to knock out the fleeing skulk.
The stalker takes a potion from a pouch and quickly swallows it 9, he is looking a little bit more lively. He then moves through the square with Shadow's helpless body, so that he is flanking Laladak with the dark creeper. When he is in position he attacks Laladak with his Shortsword Attack: 25 Damage: 22. Laladak falls to the floor, he is stable but poisoned for 1 hour, even after regaining hit points, and is paralyzed while poisoned in this way. As Laladak falls he gives the dark creeper a command in a strange language.