Pinion's docile demeanor shifts when the undead abomination teleports and unleashes the necrotic cloud.  Pinion draws his automatic pistol and narrows his focus between the creature's eye sockets. (BA: kensei shot)

shot #1: Atk 15,  15 magic

shot #2: Atk 19, 12 magic

Adams will also take a shot

shot #1: 24 to hit with 11 damage
shot #2: 14 to hit with 11 damage

The creature has taken a LOT of damage... but it doesn't seem to care as it continues its insane cackling...

Initiative:
Eppie
Toa
--
Undead abomination - 94 damage
--
Erik
Simon
Adams
Pinion

Top of the round, Eppie and Toa are up!

Toa glances up at the undead horror and the island warrior's teeth widen in a grin to match the creature's cackling.  "Ko oku tupuna," he intones solemnly.

In a process Toa comprehends only instinctively, spatial dislocation actuators activate within his cerebral cortex and both he and echo-Toa teleport, swapping places with each other.  So echo-Toa is now once more right next to the enemy and Toa himself is 35' away (30' from his echo).

Once more, Toa attacks remotely from his echo's position, bringing his maul around, then pivoting for a backhand blow (attacking Recklessly).

Movement:  Toa spends 15' of his movement to teleport and swap places with his echo.
Bonus Action:  As part of Manifest Echo, Toa and his echo both teleport, switching places with each other so echo-Toa is in melee (5') and Toa is 35' away.
Action:  Still Raging, Toa attacks the undead horror Recklessly from his echo's position.

Maul attack 1 to hit:  24
Maul attack 1 damage:  18 bludgeoning

Maul attack 2 to hit:  27
Maul attack 2 damage:  17 bludgeoning

On the first hit, Toa's ancestral spirits once again distract the undead horror so its attacks against all targets other than Toa (including against echo-Toa) are made with disadvantage.  In addition, any target other than Toa hit by the enemy has resistance to all damage dealt.

Attacks against Toa himself (but not against his echo) are at advantage due to his attacking Recklessly.  Toa can make AoOs from echo-Toa's position.

Muttering to herself, Eppie fires again with her infused revolver at the undead monstrosity.

To hit: 19
Damage: 9

The monster is still 'alive,' for the moment.

It's going to take a swipe at echo-Toa (with disadvantage)

Claws vs. echo
Attack: 17; Damage 10 (piercing) + 5 necrotic.

Result of the above determines what it does next.

Failing to connect, it risks an attack of opportunity to rush Toa. If the echo's AoO doesn't kill it, it attacks the tribal warrior with its second claw attack. I believe the attack is with advantage from Toa's reckless attack, unless someone else did something to it to cancel that out as well.

Claws vs. Toa
Attack: 27 Damage: 18 (piercing) + 9 (necrotic) = 27

Simon will take another shot and a Psychic blade. 

Attack w/ Revolver: 16 Damage: 13

Blade Attack: 22 Damage: 10

Sneak Damage: 15 

Erik: Round 2

Erik continues to fly above the courtyard. Now that the undead abomination has somehow teleported to stand near Toa -- one of the Toas, Erik flies a little higher and circles around to get a clear shot at the creature.  As it rushes to the "other" Toa, Erik again points at the creature and, twice more, a bolt of electricity arcs from his hand toward it.

Movement: 35' flying, 10' to increase altitude to 30' and up to 25' to circle around toward the side or back of the creature.
Action: Two attacks with Lightning Launcher, range (90/300)
  Attack 1: 16, Damage: 10 (lightning)
  Attack 2: 23, Damage: 8 (lightning)
  If either attack hits, the first hit does an additional 6 (lightning) damage (or 8 (lightning) damage if it was a critical hit)
Free Action: None.

Toa tries to slam the undead horror with his maul opportunistically as it moves away from his echo, but is a bit slow to react:

Maul AoO to hit:  11
Maul AoO damage:  16 bludgeoning

As the creature's claws strike him (indeed with advantage due to Reckless Attack), the spirits of his ancestors absorb some of the blow.

Rage gives Toa resistance to piercing damage, so he takes (24 / 2) + 17 = 29 total damage. 

Since the AoO missed, Erik gets the finishing blow. With one last insane giggle, the creature falls... and as it does, the shadows seem to wrap around it, to consume it. Then it is gone, as if it was never there at all. The courtyard is silent again... but who knows for how long? What else might emerge from the catacombs if you stay here?

Toa stands over where the corpse of the undead horror should have been, had it not vanished in shadow, both he and his echo looking flummoxed.

"Why all the cackling?  What was so funny?"

Suddenly, something occurs to the island warrior and he looks around surreptitiously in a panic before breathing a sigh of relief to see Pyx unscathed.  It appears Toa is becoming the tiniest bit attached to the sauro-avian creature, though he would be loathe to admit it.

His concerns for Pyx's well being allayed, Toa stretches broadly and grins.  Second Wind:  heals 13 hp.

"All of you fight well.  The spirits of my ancestors will protect you.  Where do we hunt next?"

Quote from Tybard>>

Toa stands over where the corpse of the undead horror should have been, had it not vanished in shadow, both he and his echo looking flummoxed.

"Why all the cackling?  What was so funny?"

With the creature gone, Erik lands in the courtyard. Snow scatters away from his boots as he lands, and his helmet opens up to reveal his face.  In response to Toa, he says in a confident, almost boastful, tone, "Well, personally, I thought that it was funny how quickly we defeated it!  But seriously...that didn't sound like a laugh of humor to me.  It seemed more like a laugh borne of insanity.  Perhaps that thing was once a man and was driven insane by whatever process transformed it into that."    

"All of you fight well.  The spirits of my ancestors will protect you.  Where do we hunt next?"

"Next?", Erik says incredulously, "Please tell me that you have a way to get away from here and that you weren't planning to stay here overnight."  He looks around the group and explains his thinking, "Mr. Spikey Longfingers looked terrifying."  He speaks briskly and doesn't sound afraid, but as he continues, he repeatedly casts nervous glances toward the door down to the catacombs.  He says, "But I wouldn't want to wouldn't want to face more of them.  Especially not after nightfall.  Even if the Dark Lord and his allies have left this place, who knows what other abominations he summoned here after his release!"

Once someone reminds Erik that they're supposed to be looking for survivors of the first expedition, he volunteers to help them check the bodies in the courtyard and check off names on his roster of the members of the first expedition.  He hopes for his colleagues' sake that he's not the only survivor, but if there are no other survivors, he hopes that they find everyone's bodies quickly. Otherwise, they might have to search the whole facility -- and maybe even the catacombs! -- to look for anyone they haven't found.

Eppie holsters her revolver with a frown as Vox IV belatedly comes trotting up to her from where it had hung back with the other group before they burst through into the courtyard. Pyx is still wrapped around Eppie's shoulders like a feather boa, looking a bit green and worse for wear after the poison cloud encounter. She chirps, and it comes out sounding a bit like a burp.

"Agreed," Eppie says. Squinting at Erik through her goggles, she asks, "You're the fellow that was chained up in that room, eh? Good, I'm glad Vox IV was able to free you. Now, you are quite correct. As soon as we find any other survivors, our orders are to evacuate immediately. Shall we proceed with our search?" Eppie digs around in Vox IV's saddlebags and whips out a pen and sheaf of paper, and a ruler (Cartographer's tools, one of the only tools she isn't proficient in). "Anyone want to make a map as we go?" Holding the tools in one hand, she fiddles with her goggles with the other. "And don't you worry about it being dark when we're exploring the basement. These goggles are special. I can see anything with these, as long as it's close enough. Even if it's invisible. We'll find them!" She seems unperturbed by the fact that if something is close enough for her to see it, it's probably close enough to kill her, too.

Pyx snuffles a bit, then hops down from Eppie's shoulder and starts sniffing around the snow where the shadowy creature disappeared.

Pyx Perception: 24

Random dice roll for no reason: 8

Pyx doesn't notice anything special about the spot where the creature disappeared... it's just a patch of ground.

So, what is the party doing now? Continuing exploration of the fortress, taking some time to rest... or descending into the catacombs?

(Can we leave through the helicopter? I thought they were blowing this place up.)

(If you call it in now, it will take about an hour for the chopper to return)

(Are we all together again?) 

(yes)

Simon will say, "We should call in our exit and secure the room, this is more than we bargained for."