Ordin Con save: 12

Riris steps inside the room where the fight is mostly focused on (K-16),  and she plays a few notes on the lute she sneakily grabbed as soon as the prospect of a fight was made clear, to get out a casting of Protection from Evil and Good on herself this time. She adds a little tune on the lute just to give Mixum some inspiration before putting the Lute away and approaching the Keleton (N-17) with most likely the intent to draw her rapier next.

Mixum twists away from one of the Boneless bites as Charlotte attempts to guard him from attack. Charlotte uses her action to disengage from Boneless and back ten feet away so that they are position between Riris and Odrin. Mixum draws his self loading/repeating hand cross bow and fires twice (action / BA)

attack:  25

damage: 7

attack:  14

damage: 9

(Mixum moving to L-17) approx.

Ac-Keleton Multi-Attack:
Claw vs Riris(AC 15) - Attack: 14 Damage: 4 slashing.
Claw vs Riris(AC 15) - Attack: 14 Damage: 9 slashing.

Pip quickly casts spell on Ordrin seems to elicit some revulsion from the writhing mass of skin to the north. Yet ran as far to the south in the room as his legs and space allotted, he catches as well the curious lack of the same disgust or wariness in the Shadow Wolf. Rather, it appeared more torn between menacing Ordrin and glowering at Riris' light.

Mormesk remains unseen and unheard for the moment. A boon for the party, or an ill omen...

Riris slips into the room with lute in hand, strumming the notes upon its mystical strings that would offer her a like protection as the one afforded Ordrin. But as she carried on further inside, so too would nimble fingers continue to dance upon the strings in a little tune just to get Mixum's head bobbing a bit to the beat. 

Mixum takes no further chances and immediately bids Charlotte to get some distance

Keleton's eyes, tiny embers barely visible at first, grew into more robust embers seeing the bard daringly close to its being. "Foo... lish." It rasps in a voice similar to that of Mormesk himself, before attempting to rake Riris twice with its sharpened fingertips.

The sound of bones rattling can still be heard echoing from the south as the Undead Reinforcements(S) draw ever closer to the fight.

Ordin is now up! 

Initiative Tracker(Round 1):
Lair Action - 20
Undead Reinforcements(N) - 20 (Appearing in ???)
Boneless - 20
Shadow Wolf - 16
Pip - 11 (Concentrating on [Tooltip Not Found] on Ordin.)
Mormesk - 11
Riris - 6 (Concentrating on [Tooltip Not Found] on self.)
Mixum - 3 (Bardially Inspired)
Keleton - 2
Undead Reinforcements(S) - 1 (Appearing in ???)
Ordrin - 0

Key: 

M=Mixum
O=Ordrin
P=Pip
B=Boneless
K=Keleton
S=Shadow Wolf
R=Riris
E=Mormesk
U=Undead Reinforcement
D=Undead Reinforcement

 Updated Map:

 

Seeing the tide of evil upon them, Ordrin wraps a leather cord around his hand, from which dangles a talisman of Moradin, and professes the glory of the All-Father to friend and foe alike. At the same time, a spectral form vaguely resembling his lost companion Firnat materializes behind the shadow wolf. Then he brandishes Lightbringer at the shadow wolf and both he and the spectral warrior strike it with Moradin's might.

Lightbringer attack: 22; damage: 4
Spell Spiritual Weapon attack: 9; damage: 5

Lair Action: Dreadful Mire
Non-Undead Creatures have their movement speed halved, and receive half-healing.

Ma: 14-K.
Ac-Boneless Multi-Attack:
Slam vs Ordrin(AC 20) - Attack: 18 Damage: 4 nonmagical bludgeoning. 
Slam vs Ordrin(AC 20) - Attack: 8 Damage: 4 nonmagical bludgeoning. 
If both hit, Ordrin is considered Grappled. 

Ma: 15-M.
Ac-Bite vs Ordrin(AC 20) - Attack: 7 Damage: 9 nonmagical piercing.
If hit, make a Strength Save(DC 13) or be knocked Prone.

Ordrin spares but a moment to coil a symbol of faith about his hand, and assures the glory of Moradin would be with them. And as if in response to that declaration, more than specter, but for a brief few moments he is assured Firnat materialized in the flesh behind the wolf, if albeit radiating a divine light. Surprised if not in awe, the blow he lands upon the wolf next isn't quite as square on the beast skull as hoped. But it leaves the beast wary enough to throw itself more heavily out of the way of Firnat's strike. In spite of such failings, the Firnat just laughs before -- in the blink of an eye -- she'd become the spectral warrior that everyone else had been seeing up to that point; bearing but a ghost of a tired smile.

"Not Bad... Not bad at all, my little ungrateful guests... But tempted as I am to let you keep playing... it... it is time you come to have a taste of what awaits those that defy MY will!" Everyone again here's the voice of Mormesk echoes throughout the room, and exuding such force as to live the living feeling as if they're being crushed on all sides. When that feeling doesn't subside long after the sound diminishes, the party begin to notice the ground seem to take on a muddy texture, smell a fetid scent drift up their noise, and some ways swear they can see out from the corners of their eyes spectral, bony hands reaching out from the newly conjured Dread Mire and cling to their form.

As if that wasn't enough, from those closest to the door can swear they hear the shuffling of many feet and odd groans as Undead Reinforcements(N) draw ever closer to the fight from the north.

The Boneless mass, though upset in seeing Mixum makes good his escape, turns its empty eye sockets upon Ordin. Its mouth than distorts into a distorted grin as it lurches forth, fighting against the arcane ward surrounding the cleric in its attempts to pummel and encapsulate the dwarf to likely little avail! 

The Shadow Wolf, still a bit dazed by Ordrin's blow, eventually snaps its jaw back into place as it shook its head. Then with an angry snarl at the priest, it lurches back only to leap forward again in an attempt to maul and force Ordrin to the ground in throwing its full weight into the lunge.

Pip is now up! 

Initiative Tracker(Round 2):
Lair Action - 20
Undead Reinforcements(N) - 20 (Appearing in ???)
Boneless - 20
Shadow Wolf - 16
Pip - 11 (Concentrating on [Tooltip Not Found] on Ordin.)
Mormesk - 11
Riris - 6 (Concentrating on [Tooltip Not Found] on self.)
Mixum - 3 (Bardially Inspired)
Keleton - 2
Undead Reinforcements(S) - 1 (Appearing in ???)
Ordrin - 0

Key: 

M=Mixum
O=Ordrin
P=Pip
B=Boneless
K=Keleton
S=Shadow Wolf
R=Riris
E=Mormesk
U=Undead Reinforcement
D=Undead Reinforcement

 Updated Map:

 

"Eek!"  Pip calls a bit overwhelmed by the ferocity of the attackers.  He focuses on his next move and points at the Boneless thing and the sound of a dolorous bell fills the air around it for a moment. The target must succeed on a Wisdom saving throw (DC14) or take 4  necrotic damage

Boneless Wis Save vs Pip[DC 14]: 2.

Ac-Mormesk Life Drain:
Life Drain vs Pip(AC 16) - Attack: 16 Damage: 25 Necrotic Damage. Con Save[DC 14] or hit point maximum reduced by same damage damage. 
Also if hit, make a concentration saving throw.

Pip startles, and yet finds the wherewithal to bend the weave yet in his favor to create the echo of death's tolling bells. Although a being no doubt kept together by the tenuous bond of necrotic energy, the Boneless creature lets out a choked cry of pain as it clawed at its own fleshy flaps in trying to keep out the noise. No matter what resistance it may muster in the end, Pip could smile with some confidence in knowing he put the fear of death in the thing.  

Yet any spark of self-assuredness kindled in that moment is bound to be tested shortly after as the air grew unnaturally cold around him. "My turn~" He hears whispered right in his ears just moments before Mormesk's rotted and translucent hand appeared to materialize from nothing at his side trying to punch him. Even in a grazing blow, the moment skin met spirit, flesh would immediately start to decay. "Ready to reconsider?" He asks after of all as the rest of him slowly appears to ripple into view and reveal the purple robed spirit floating inches off the ground. Who apart from his arms and hands appeared to be made of some red mist emanating from the shadows of the robe.

Riris and Mixum are now up! 

Initiative Tracker(Round 2):
Lair Action - 20
Undead Reinforcements(N) - 20 (Appearing in ???)
Boneless - 20
Shadow Wolf - 16
Pip - 11 (Concentrating on Protection from Evil and Good on Ordin.)
Mormesk - 11
Riris - 6 (Concentrating on Protection from Evil and Good on self.)
Mixum - 3 (Bardially Inspired)
Keleton - 2
Undead Reinforcements(S) - 1 (Appearing in ???)
Ordrin - 0 (Under the effects of Protection from Evil and Good of Pip.)

Key: 

M=Mixum
O=Ordrin
P=Pip
B=Boneless
K=Keleton
S=Shadow Wolf
R=Riris
E=Mormesk
U=Undead Reinforcement
D=Undead Reinforcement

 Updated Map:

 

Pip Con saving throw 14

2nd throw due to damage 7

Pip feels the brush of the touch upon his skin and feels the life drain from him.  He does his best to hold onto consciousness but his concentration on his spells fall as he crumples to the ground.  Not dead, not yet.  But very close.

Mixum dismounts from Charlotte and fires two (magic) bolts at Mormesk. He has Charlotte move across and in front of Pip. Charlotte is dodging but will use her reaction to guard Pip.

attack 11

damage 8

attack 14

damage 9

"Hang in there!" Those are all the words that Riris uses as a source of her incoming spell targeting the fallen Pip.

Healing Word: 4

At the same time, as she's trying to bring Pip back into the fight, Riris is drawing her rapier, which is when she decides that she's too close to the Keleton to move away so she can help better on the other side. She does have a distraction, though. A short stab is all she'll have to do. (Using the Mobile feat to get away from an Attack of Opportunity)

Attack: 18 Damage: 6 Piercing

Hit or miss, that should be enough, so she takes two steps back (M-20) to close in on Mormesk, whom she raises her blade towards. "Ya better leave him alone!"

Following the End of Riris Turn:
Mormesk Legendary Action: Enervating Grasp vs Riris(AC 15) - Attack: 7 Damage: 3 cold damage. Target speed is reduced by 10ft until the end of their turn.

Ac-Keleton Multi-Attack:
Claw vs Charolette(AC 15) - Attack: 8 Damage: 8 slashing.
Claw vs Charolette(AC 15) - Attack: 5 Damage: 8 slashing.

Riris' words, powerful though they may be, would feel distant to Pip. Yet dampened as the magic behind them was by whatever dark magic infected the area at the moment, Pip gasps to life. A precision stab to the spine throws the 'Keleton' off balance enough for her to slip away after, only to stand up to Mormesk, blade raised and a demand spoken with conviction. Mormesk chuckles as he turns more to face the halfling. "As you wish." 

Mixum dismounts to send Charolette off to defend Pip to the best of her ability. Although as he fires off bolt after bolt from ground level, only the one so much as grazes the cloak of the spirit, drawing at least some attention. Attention that proves to be fleeting however, as he soon appears to focus again on Riris.

"But such bravery has a price." Mormesk states as he abruptly reaches out as if to grasp Riris. Yet unlike with Pip, his hand would actually phase through whatever he might make contact with it, and chill everything about it so much, frost would actually start to form and restrict movement. (Legendary Action)

It balance eventually restored with a bit of time and physical adjusting, the Keleton appears to march over to Riris. But though it raised a hand to strike the halfling, a sharp gesture from Mormesk gives it pause for one moment, before nodding and turning its deadly fury upon the mechanical trouble maker that was Mixum's mount.

As the fight yet continues, the rattling from the south grows ever louder. The group couldn't have more than a few rounds by the sounds of it before Undead Reinforcements(S) reached the chamber.

Ordin is now up! 

Initiative Tracker(Round 2):
Lair Action - 20
Undead Reinforcements(N) - 20 (Appearing in ???)
Boneless - 20
Shadow Wolf - 16
Pip - 11 
Mormesk - 11
Riris - 6 (Concentrating on Protection from Evil and Good on self.)
Mixum - 3 (Bardially Inspired)
Keleton - 2
Undead Reinforcements(S) - 1 (Appearing in 3 Rounds)
Ordrin - 0

Key: 

M=Mixum
O=Ordrin
P=Pip
B=Boneless
K=Keleton
S=Shadow Wolf
R=Riris
E=Mormesk
U=Undead Reinforcement
D=Undead Reinforcement

 Updated Map:

 

 Ordrin slams the glowing mace into the shadow wolf again, at the same time directing the spiritual visage of Firnat to assault Mormesk (else Keleton if not enough movement).

Ordrin attack: 25, 6

Firnat attack: 20, 6

 OoC: sorry for the messy post. my rolls didn't auto-calculate crit damage, btw.

Ordin: Crit Bonus Damage: 2.

Rc-Mormesk: Casting Shield vs Firnat(20), raising AC to 21.

Following the End of Ordrin Turn:
Mormesk Legendary Action: Enervating Grasp vs Riris(AC 16) - Attack: 22 Damage: 14 cold damage. Target speed is reduced by 10ft until the end of their turn.

Lair Action: Feast on Life
All Creatures within 10ft of Boneless, Mormesk, and Keleton (excluding their allies) must make a DC 11 Con Save or take 4 Necrotic Damage.
So basically everyone needs to make the save.

Ma: 14-K.
Ac-Boneless Multi-Attack:
Slam vs Ordrin(AC 20) - Attack: 20 Damage: 7 nonmagical bludgeoning. 
Slam vs Ordrin(AC 20) - Attack: 25 Damage: 11 nonmagical bludgeoning. 
If both hit, Ordrin is considered Grappled. 

Ma: 15-M.
Ac-Bite vs Ordrin(AC 20) - Attack: 12 Damage: 7 nonmagical piercing.
If hit, make a Strength Save(DC 13) or be knocked Prone.

Ordin, in seizing a moment of distraction for the shadow wolf, cracks it hard across the jaw and knocks a few teeth out in the process. Something the spirit of Firnat might've done as well to Mormesk -- if the appariation even had teeth -- if not for a hissed out arcane word from the undead conjuring a spectral purple barrier of energy to intercept the blow. 

"Such a stubborn spirit! But, I will not be denied forever." Mormesk states as he again swept a chill hand for Riris' form. (Legendary Action)

Lair Action: Just as the environment itself appears to revert back into a less supernatural state, those close to the undead entities feel the very air grow thick and crushing as if some malevolent force tries to squeeze the very vitality from them. And those whose body cannot withstand the pressure find themselves spitting some strange globule of energy that immediately zips into the nearby horrors, eliciting a choked and sinister laughter.

Those expecting to hear more shuffling and groans drawing closer from Undead Reinforcements(N) are instead treated to the stranger sounds of shouting in the distance, alongside a few other odd noises. Could the enemy forces expected have been stalled in the party's favor?

The Boneless mass at least hardly cared for anything going on beyond the room. Instead it continued to lash out at Odrin with a renewed fury now that the barrier of Pip's magic had come undone in the gnome's previous fall.

The Shadow Wolf, though quite a bit roughed up looking after the last blow, remained all the more determined to get its revenge in another attempt to bear the dwarf down to the ground while mauling him to pieces. 

Pip is now up! 

Initiative Tracker(Round 3):
Lair Action - 20
Undead Reinforcements(N) - 20 (Appearing in ?!?)
Boneless - 20
Shadow Wolf - 16
Pip - 11 (Concentrating on Protection from Evil and Good on Ordin.)
Mormesk - 11
Riris - 6 (Concentrating on Protection from Evil and Good on self.)
Mixum - 3 (Bardially Inspired)
Keleton - 2
Undead Reinforcements(S) - 1 (Appearing in 3)
Ordrin - 0

Key: 

M=Mixum
O=Ordrin
P=Pip
B=Boneless
K=Keleton
S=Shadow Wolf
R=Riris
E=Mormesk
U=Undead Reinforcement
D=Undead Reinforcement

 Updated Map:

 

Mixum con save  24

takes 3 necrotic

After avoiding Mormesk's grasp not once, but twice, Riris can't help but shoot a cocky grin in the direction of her attacker. "Ya talked about a price, but you're doin' a really bad job makin' me pay it, huh?"

Her happiness does get interrupted by the supernatural pressure of this place, but if she could shrug it off and more importantly, maintain concentration on her spell, then no problems still.

Constitution Save: 12

Ordrin CON: 13

A sudden loud ringing noise, painfully intense, erupts from the spot at O16 and Keleton and the Shadow Wolf need to make a DC14 Con save for 14 thunder damage on a failed save, or half as much damage on a successful one.

Shatter

I believe I placed the spell to only get those two.