Bryony releases the arrow she had nocked after she sees Mormesk unleash some sort of horrible magic against Jurgen. 

Shortbow: Attack: 14 Damage: 6

Draco jumps in to defend Jurgen. 

(Disadvantage roll): Life Drain Attack: 19. Hit: 20 necrotic damage and Jurgen should roll a Constitution saving throw. 

Bryony manages to hit the protruding skull face with her well-aimed shot, although it seems to do less damage than it ought to have. (Resistance to the damage from her bow.) It shows only a mild mark where the arrow hit it.

Ruven may make his readied magic missile attack before it vanishes behind the door. 

Boona may also act, although by now the ghostly figure has vanished from view behind the broken door.

As soon as the thing attacks, three bolts shoot from Ruven's outstretched arm and strike its head:

10 force damage

Jurgen CON save 24

(Jurgen still takes the 20 necrotic damage, but suffers no additional effects, as he feels his life force being pulled out of him.)

The side of its translucent skull singed by Ruven's magical darts, the undead apparition retreats back inside the stone building, somewhere to the other side of the stone door.

Boona may get an opportunity attack against it (albeit at 3/4 cover) as it does so.

Then, Boona may have the rest of her turn, followed by the rest of you.

Boona AoO- Attack: 21 Damage: 11

Boona rears back and strikes at the doors with her Maul.

Attack: 15 Damage: 12

Boona manages to strike its semi-transparent skeletal hand as it retreats, doing minor damage to Mormesk's creepy appendage.

Then, Boona whacks the damaged stone door with a mighty BANG, but does little to damage it. (AC higher than 15.)

Quinn, Bryony, Ruven, and Jurgen may all act, although your foe is no longer visible. 

Jurgen says while bandaging his significant wounds "We should let him be. I believe he could have caused us a lot more pain than he did. Let's continue on."  

Bryony nods and moves toward Boona. "It's ok, Boona. Let's leave him alone for now."

Quinn backs up slowly and listens for movement from skeletons or any undead further down the tunnel.

Perception check: 18

”Boona. Leave the door. Everyone hush. Does anyone hear anything moving?”

Quinn stows his shield, draws his bow and scans the room and the corridors for any threats. 

Ruven goes through undead in his mind, trying to figure out which one this creature was. It must be guarding something important...could this be the way?

arcana 15 (If it's a nature or religion check to see what kind it is, that would take 2 or 4 off the roll respectively)

Jurgen says "I could really use a short rest. But I would suggest not hear near grumpy" (down to 9 HP)

"Thieves!" growls the hoarse, unearthly, whispering voice within. "Barbarians! Trespassers. Freebooters! Looting plunderers..." It pauses, then raises its eerie voice. "You want loot? Coin? Magics, perhaps, strangers? You want to kill? ...KILL the guard of the Forge. The other building, up the escarpment. KILL the guard. We will share what it guards..."

Ruven recognizes this entity as a "wraith" – the incorporeal remnant of a particularly hateful being. Wraiths can transform those they have slain into spectral undead servitors. The creature is suffused with negative energy, and its mere passage through the world leaves nearby plants blackened and withered. Animals flee from its presence. Even small fires can be extinguished by the sucking oblivion of the wraith’s horrifying existence. A wraith can move through solid creatures and objects as easily as a mortal creature moves through fog.

The "starry" cavern is quiet, as is the escarpment to the north, and the fungi cavern to the south. 

Boona nods to Bryony and Quinn and moves away from the door to wait for new instructions.

"Lets continue to explore north." Jurgen suggests

Barbarian?! Plunderer!? Ruven splutters, incensed. I'll have you know, good sir, I'm the foremost authority on Wave Echo Cave and the Forge of Spells at the University. I am no barbarian. Ruven stomps off to the North, muttering sullenly. Sharing, indeed. I shall share a few more missiles with him.

Bryony shakes her head, glancing back at the door. She catches up to Ruven as everyone heads toward the northern part of the cavern.

"What is that ghost's problem?" she mutters. "We are obviously not barbarians or plunderers. I mean...do we really look like we are?"

She glances down at herself, suddenly worried that she looks like a bandit or hooligan of some kind.

Not a ghost, little halfling, a wraith Ruven says, calming down. They are particularly hateful and vicious, and more than likely the causation of the belligerent undead we were previously forced to contend with. We shall be forced to deal with it at some point, I am sure, but perhaps when we are all more ready and equipped for it. 

Ruven will lead the way up the stairs.

Ruven climbs the stone stairs cut into the escarpment. More cavern, more bones. Another freestanding stone room. Another set of damaged double doors, scorched and cracked, but standing slightly ajar.