There is a pained mooing just within the fog, and the cow with its head poking out is looking panicked.

There is more shifting around on the stairs as Neth takes Kolose moving past him as a retreat and follows suit.

Initiative:

Neth: 23 
Stena: 22  <-----<<
Petmailer: 20  <-----<<
Lia: 20 
Cows: 19
Yuk Yuk: 19 (readied dagger throw) (up the stairs)
Spiderbait: 14 (readied dagger throw) (up the stairs)
Kolose: 13
Galingal: 12
Fargas: 12 (readied hand crossbow) (up the stairs)
Jimjar: 8 (readied hand crossbow) (up the stairs)
Coffin Residents: 7 (No longer Turned/No longer Turned/No longer Turned/X)

"If they chase us all the way up, we can hopefully take them out cleaner up there- I mostly don't want to get hit with any particularly bad magic," he says, seeming to understand the crystals are very bad news, even if he can hardly put his fingers on the "why" exactly. 

Stena moves out of the way should the cow bolt up, and holds an attack for any ghosties.

"Move. Call when yer in place."

Petmailer uses one of his arrows on one of the trampling cows who are bolting, to prevent being run over.  He targets one that is threatening to overtake him and his looses an arrow at it, hoping to put it down.  Afterwards, he moves 30 feet higher up the stairs.  

Long bow : 18 to hit, for 11 points of piercing damage.  

((Did you roll with advantage for being invisible? Probably doesn't matter much, it's a cow, but in case you crit?))

(2nd roll 11, did not crit)

Petmailer shoots the cow in the face and it drops atop the cow that Stena had previously dropped at the edge of the fog as he suddenly becomes visible again before falling back up the stairs.

Another bovine head pushes out of the fog, mooing and unsure, before making to leap and clamber over the two dead of its kind that are currently strewn across the doorway.

It collapses mid scramble, tumbling to slam against the stone wall.

No more mooing can be heard...

Initiative:

Neth: 23 
Stena: 22  
Petmailer: 20
Lia: 20 
Cows: 19
Yuk Yuk: 19 (readied dagger throw) (up the stairs)
Spiderbait: 14 (readied dagger throw) (up the stairs)
Kolose: 13  <-----<<
Galingal: 12  <-----<<
Fargas: 12 (readied hand crossbow) (up the stairs)
Jimjar: 8 (readied hand crossbow) (up the stairs)
Coffin Residents: 7 (No longer Turned/No longer Turned/No longer Turned/X)

"These wraiths very slow," Galingal says. 

He considers casting a spell into the fog but decides it could be a waste of valuable energy. Best to wait until they can see their enemy.

Galingal backs up a couple of steps, his empowered rapier held at the ready. 

Kolose continues doing a standard shift further up, but this time keeps an toll the dead ready just in case he should see something.

"...You know, with some of us having gone invisible, maybe they figure we disappeared."

"...Wait, the invisibility magic would not have caught the specters as well, would it?"

Kolose muses, and hears a "I'm note certain, but it bodes ill for us if it did..." from Neth.

Galingal falls back to just in front of the invisible Stena, and none too soon, as the first of the undead emerges from the fog, followed by the two others. They immediately orient on the shadow elf, and surge forward.

((For positioning of the others.))

((Neth is invisibly on the landing behind Petmailer, closer to Stena, and Kolose has invisibly moved past Lia and is halfway to the next landing (almost directly above the undead, just a few flights up. There would be crystals studding the walls providing dim light throughout the stairs once you got up above the first landing or so, I just didn't put them in because I didn't anticipate we'd need them on the stairs.))

Initiative:

Neth: 23 
Stena: 22  (Ready to shoot)
Petmailer: 20
Lia: 20 
Cows: 19
Yuk Yuk: 19 (readied dagger throw) (up the stairs)
Spiderbait: 14 (readied dagger throw) (up the stairs)
Kolose: 13  (Ready to Toll)
Galingal: 12  (Ready to Stab)
Fargas: 12 (readied hand crossbow) (up the stairs)
Jimjar: 8 (readied hand crossbow) (up the stairs)
Coffin Residents: 7 (No longer Turned/No longer Turned/No longer Turned/X)   <------<<<

Stena fires,

Attack: 27 Damage: 9

The arrow slows as it snags in the undead, but passes through and clatters to the stone floor beyond.

Galingal stabs out at the closest ghost/wraith/spectre (if more than one in melee range, he uses his reaction against the one Stena fired an arrow at).

Using the still magically empowered rapier, Galingal stabs forward.

Reaction - Rapier - Attack: 8 Damage: 5  (magical piercing damage) 

Stena and Galingal focus fire on the lead undead, it seems pained by Galingal's strike in particular, but pushes on, rearing back to swipe at the cleric as the invisible Neth further up on the stairs is just able to get visual on it to unleash an eldritch blast.

"This is BAD!", the tiefling moans as he becomes visible again.

Attack: 13 Damage: 9

56

"This is REALLY bad!", he exclaims, as his fingernails begin growing at a visible rate, already a few inches long in a matter of seconds.

However, his attack, combined with the rapid blows of Stena and Galingal, has the undead lose cohesion as it shrieks and drifts into shreds of wispy  ethereal matter that soon vanish as the other two swarm towards Galingal, mouths agape as they swing at the cleric.

Attack: 17 Damage: 7
Attack: 22 Damage: 11

((Galingal, please make a DC 10 CON save on the one that hit you, if you fail, your Maximum HP is reduced by 11 as well.))

One of them rakes at Galilngal, and he can feel the flesh where he is struck grow warm and pulsing, straining against his armor. The other misses as he pulls back, but the pair of them whirl and dive through the walls, leaving Galingal and Neth in the lower part of the stairwell with a currently invisible Stena.

((Gal's reaction was used for the readied attack, so no opportunity attack possible.))

Neth, his fingernails continuing to grow, moans about how things are NOT fine, and continues to fall back up the stairs up past Petmailer, struggling with his elongated nails to make the proper motions to start another casting of his spell.

Initiative:

Neth: 23 (Readied EB)
Stena: 22     <------<<<
Petmailer: 20    <------<<<
Lia: 20 
Cows: 19
Yuk Yuk: 19 (readied dagger throw) (up the stairs)
Spiderbait: 14 (readied dagger throw) (up the stairs)
Kolose: 13  (Ready to Toll)
Galingal: 12  
Fargas: 12 (readied hand crossbow) (up the stairs)
Jimjar: 8 (readied hand crossbow) (up the stairs)
Coffin Residents: 7 (No longer Turned/No longer Turned/X/X)

Stena will remain put and silent, bow ready for when the ghost's reappear.

Petmailer moves to a portion of the stairwell where he can see what is going on below.  He is looking in front of Galingal, trying to see the ghosts as they are approaching him.  He remembers one of his tricks to outline enemies and he partway closes his eyes, points at the ground beyond Galingal and casts faerie fire, hoping to outline the ghosts and make them easier to hit.  He makes sure that Galingal is not included in this, but the area beyond him where the ghosts are.

Faerie fire (once per LR) : DC 11 Dex save or : 

Each object in a 20-foot cube within range is outlined in blue light. Any creature in the area when the spell is cast is also outlined in light if it fails a Dexterity saving throw. For the duration, objects and affected creatures shed dim light in a 10-foot radius.

Any attack roll against an affected creature or object has advantage if the attacker can see it, and the affected creature or object can't benefit from being invisible.

((They've dipped out through the walls, but he can hold it to drop on them them if/when they re-emerge.))