Komos was just about to ask what was so creepy when he sees the creature blink out of vision and suddenly understands.

Initiative: 13

Tiamat: 19

The Grey: 19

 Komos's turn.

Not sure what the threat is exactly, Komos instinctively pulls out his Icosahedron and pushes two buttons. The first causes his cannon to ring its bell, giving everyone 10 temporary HP. The second, rings another, very soothing bell from inside the Icosahedron blessing everyone in the group.

Komos quickly puts down measures to support his allies.

Varn's turn.

A creature, or was he imagining things. In here, there's no way to tell. But Komos reacted to it, so the former option seemed more reasonable. But without seeing the enemy... 

Varn raises his wand, focusing his magic at the tip, and proceeds to wait. If the enemy reveals itself, he'll blast its mind with disorienting energy. 

Readying Mind Sliver, if the enemy is seen in range (60'). If triggered, the enemy must succeed a DC 17 Intelligence save or take 11 psychic damage, and suffer a 4 penalty on its next save before the end of Varn's next turn.

Creature is immune. Attacks Tiamat.

Attack: 15 Dmg: 31

Tiamat attacks back.

Great Sword +2: 11 Dmg: 11
Great Weapon Fighting: Reroll if 1s or 2s in above damage roll: 6

Forgot Extra attack: 10 Dmg: 13

Great Weapon Fighting: Reroll if 1s or 2s in above damage roll: 10

The lights flicker. The room goes dark and for a second the lights come back on. The creature can be seen standing right in front of Tiamat.

Varn quickly lets loose his mental attack but it does nothing, even though he knows the spell was executed perfectly. The  creature is immune to the spell (Immunity: Psychic, Charmed, Frightened)

It plunges its sharp hand into Tiamat's chest, her armor unable to stop it. Tiamat coughs up some thick red blood. In a panic she quickly strikes at the creature. She manages to land one good slash on it but when she winds up her second attack, the lights flicker again and the creature is gone. Her sword cuts nothing but air and slams on to the ground with a clang.

Tiamat grimaces. Dark blood runs down from the hole in the left side of her chest on to the floor under her clawed feet.

The lights flicker again and now the creature is behind Tiamat.

 Skarro's turn.

Skarro, ever-faithful to Tiamat, shrieks in anger as it attacks her. Using his reaction, he strikes it when it comes withing range, then follows up with even more deadly strikes. Finally, he reverses his staff, striking the creature with the but, then looks around, wondering if there is a safer place to fight.

"Keep your backs to the wall...or crates!" he calls out, hoping to avoid being attacked from behind.

(( @DM: Is there a corner where they could group up? ))

Reaction attack (if it came within range before Skarro's turn): Attack 24; Damage: 8 + 5(acid) = 13

Main Attack 1: 8, --
Main Attack 2: 24; Damage: 10 + 2(acid)  = 12
QM Attack: 23; Damage: 10 + 3 (acid) = 13

(( @DM: I *think* Tiamat probably had 11 tempt HP ))

((You're right. I forgot about that))

 Skarro quickly manages to land a few hits against the creature. It hisses out and the smell of rotting corpses fill the room when Skarro's acidic staff makes contact. Then the creature vanishes once more with the flickering lights.

((Skarro: There are but the corners are like 150ft away because the room is really big))

Varn and Komos' turn.

Bump.

Varn and Komos' Turn

Being unable to see the target, or know where it is, really limits Varn's options. Not only his, but everyone else's too, surely. But the creature does appear when it tries to attack them, so at least momentarily, it is vulnerable. If they can kill it quickly during that brief time when it's visible... But is everyone quick enough?

It was time to try a new spell. Varn focuses on manipulating the fabric of time itself, manipulating it to his will. Bending it to flow asymmetrically. While he himself wouldn't even feel the difference, for Skarro and Tiamat, time will flow at a different pace. It is only a slight change, but it might still be meaningful. He hoped it would be.

Casting Haste on Tiamat and Skarro (via Twinned Spell).

Komos rings the his health bell again, granting everyone who needs it another 14 temporary HP. He then aims his rod, scanning the room, ready to loose a pellet when he sees the creature reappear.

ready action: 2+Attack: 12 Damage: 14
((remember Bless~))

Drop Crates. Skarro DEX 13 save 19. Become restrained and prone on fail. Also take 15 Bash damage, half on success.

Grey vs Komos:  29 Damage: 22

On hit, concentration check 20 + 1

Suddenly one of the crates seem to shake on its own and then it crashes on to Skarro. He manages to move out of the way of the massive crate just in time to avoid getting crushed under it instead just takes a bad hit to the shoulder. Parts of the crate break and spill out the machine parts from it.

The creature appears once more and rakes Komos. Komos tried to blast it but the creature effortlessly swats away his device at lets the pallet hit the ceiling. Luckily the armor and the energy shield absorb most of the hit.

Skarro's turn.

(( @DM: Skarro's HP are lower than expected...did I forget/miss something? I may not have looked for a while, but last time I checked they were full. Also his and Tiamat's DEX saves are with advantage. ))

Skarro feels strangely light on his feet as Varn's spell settles. Smiling at his newfound speed, he circles the Grey, trying to put it between himself and the pile of crates, then focuses his ire on the creature before striking again. On the last strike, his quarterstaff briefly glowing with a green light.

Attack 1: 19; Damage: 10 + 2 (acid) + 4 (necrotic) = 16
Attack 2: 26; Damage: 9 + 4 (acid) + 3 (necrotic) = 16
Attack 3: 18; Damage 11 + 1 (acid) + 3 (necrotic) + 7 (fire) (Green Flame Blade) = 22

((Regarding HP, I didn't do any actual damage this fight.I think it might be from past damage or something. Leave it as is.))

Tiamat attacks: 13 Damage: 12  If 1s or 2s: 7
Tiamat attacks2: 22 Damage: 13  If 1s or 2s: 4
Tiamat attacks3: 11 Damage: 13  If 1s or 2s: 8

Skarro dashes forward and with the high speed granted to him by Varn, Skarro makes a series of well aimed and blazing fast strikes with his staff. He moves so fast that it almost looks like there's a afterimage behind trailing behind him with each movement. While his barrage of attacks all hit, Skarro finds that his focus is not working like it usually is (Immunity: Necrotic)

Tiamat moves in from the other side and begins down her own barrage of attacks with her great sword. However when she made the final swing the creature disappeared again.

Varn and Komos' turn.