Reaction: Uncanny Dodge. It takes 4 Slash, 4 Thunder and 7 Force for a total of 15 damage from that attack.

Legendary Action: Twin Scythe Vs Gendan
Scythe1: 15 Dmg: 9
Scythe2: 15 Dmg: 11

Gendan moves up to the creature following the sounds and swings. She registers the feedback from her sword digging into the creature. It was a hit but suddenly she is pushed back as the twin scythes clang against her armor.

Runt, Deva and Val's turn.

(Alright folks, it already used its reaction. Tear into it! 🤪)

(Also, it’s prone, so melee should not be at disadvantage due to advantage/disadvantage canceling out.)

Starting Turn Necro Damage: 3.

Devarus had been struck so hard and fast that there been hardly had time to draw a breath from the first gasp of pain. He soldiered through the pain, and although blinded, could hear it scrabbling and cackling as it tried to flee once more. Any hesitation flees upon hearing more pained cries from friends and allies alike. So, he put away Scattershot in a hurry to draw the longbow.

"Turn and face me you foolish boy! I'm not done yet!" He accentuate his point by loosing a few of the magic arrows in quick succession. Each one infused with a bit of fighting spirit and grit so potent, an echo of it would try to lodge itself. For at any moment, it could be shot yet again!

Bonus Action: Catch a Second Wind for 15 HP back.

Action: Put away Scatter Shot and draw Longbow.

Adrenaline Surge!

AS Action: Attack with Longbow(using +1 Arrows).
#1 To Hit: 11 Damage: 12 magical piercing.
#2 Attack: 8 Damage: 15 magical piercing.

If #1 hits, using Goading Maneuver, inflicting 7 extra damage. Wisdom Save[DC 15] or it has disadvantage on all attacks not on Devarus.
If #2 hits, using Goading Maneuver, inflicting 6 extra damage. Wisdom Save[DC 15] or it has disadvantage on all attacks not on Devarus. 
(if 1st attempt failed, using goading again. If not, using Distracting Manuever. But, if Elsara is yet within its reach, using Maneuvering Maneuver instead(highest priority) so she can spend reaction(or if already spent, using it on Valwyrn) to move at 1/2 speed.)

((1st attack missed. 2nd attack hit))

WIS save: 12

Legendary Action: Twin Scythe Vs Gendan.
Scythe1: 20 Dmg: 9
Scythe2: 20 Dmg: 9

Crit: 3

Deva fires blindly into the darkness. He hears one of his arrows striking he ground while a deep howl tells him that the 2nd arrow found its mark. Then a scream of pain rings out from Gendan as the tip of one of the scythes hook into Gendan's thigh, finding a gap in her armor. It hurts like only a rusty scythe can.

Runt and Val's turn.

Val shootes a pray to Mother Nature and a majestic spirit deer appears next to Gendan. He aims and fires his longbow against the creature.

( BA Cast: Healing Spirit at lvl3: every ally who starts its turn or moves next to it heals 2d6. Remaining heals: 5

Action: Attack: Attack: 12 Damage: 8

Another attack roll in case it is still prone and ranges is at disadvantge: 10

Movement: Val position himself to have a clear LOS with he creature and, if possible between it and the exit. Probably not the best idea but... ) 

Edit: a natural 20 and a nat 1. Dice gods are cruel Hope it hasn't has disadvantage. ;) In case the nat 20 goes: 8

Runt rushes to her battle sisters side as she would sence her pain from the rusty scythe like attack. This enrages the petite half orc and she beings to swing her axe, hoping to cleave her foe.

BA rage 

Attack: 17 Damage: 14

Attack: 14 Damage: 16

Unfortunately it was still prone so Val's attack missed. Fortunately, it ran out of Legendary actions so it can't attack Gendan again.

For Runt, Dice rolls not showing.

Quote from ZulkRS >>

Unfortunately it was still prone so Val's attack missed. Fortunately, it ran out of Legendary actions so it can't attack Gendan again.

For Runt, Dice rolls not showing.

 
Is it not still dark? If so, I'd attribute Val's miss to the darkness, not to the proneness of the enemy. (Even though disadvantage + disadvantage = disadvantage)

(It's also dark. That too. I don't think it really matters though if he missed due to darkness or due to proneness. In the end the attack had disadvantage and it missed)

(Right, I understand that. Just wanted to make sure we knew if the darkness went away.)

Alright rolling for Runt. Assuming doing Reckless Attack because darkness would give the creature advantage against Runt anyway (after it stands up of course, which it will)

Attack1: 12 (Adv: 24) Dmg: 14
Attack2: 25 (Adv: 26) Dmg: 15

Plus extra thunder damage: 6 Thunder

Lair Action: Death Strike.

Vs Gendan: 13 Dmg: 18 Necrotic
Vs Runt: 18 Dmg: 26 Necrotic
Vs Elsara: 23 Dmg: 12 Necrotic
Vs Deva: 15 Dmg: 24 Necrotic
Vs Val: 19 Dmg: 17 Necrotic

Grim Gash Recharge: 1
Death Strike Recharge: 5

Creature stands up. No AoO thanks to darkness.

Attacks Gendan.
Attack1: 21 (Adv: 16) Dmg: 9
Attack2: 27 (Adv: 12) Dmg: 12
Attack3: 28 (Adv: 22) Dmg: 8

Runt runs to the location of the prone creature. She swings her great axe hard. The force of the impact ripples through the darkness. The creature hisses and its scythes begin to glow an ominous red. It suddenly disappears from view and all is quiet for a bit. Then from out of the inky mist, the creature's scythe can be seen arcing a trail of very dim light. It appears behind Elsara and sinks his scythe into her back, the tip coming out of her chest. The sound of Elsara gurgling blood followed by the sound of her hitting the ground can be heard. Before anyone can process what strange magic this is, the creature appears in front of Gendan and tries to hook her as well. Gendan quickly blocks the deadly attack with her shield. She could feel the dark magic flow around her as the attack is deflected. The creature reacts by reaching under the shield with its 2nd scythe and hooks Gendan in her gut. Runt moves in in an attempt to strike the creature (No AoO. Just flavor text and description) but the creature dissipates into the darkness before her axe can hit it again. It reappears above her and sinks the rusty glowing scythe into her shoulder. Deva anticipates a pattern and preemptively dodges. It pays off as the creature appears in front of him moments after and hooks its scythe into the wooden top of the shelf where he was standing moments ago. The creature disappears again and this time swings sideways at Val. Thinking fast, the elf druid grabs his bow with both hands and takes a swing. With a loud clang, the scythe is deflected away from him. The creature disappears again and then all is quiet. The darkness still pervades this room and none of you can see anything. You do not hear any sound of the creature anymore. It might have gone invisible once again and escaped. You have already seen that when it turns invisible, its sounds, though not muted, are more muffled than normal.

Elsara on Death Saves.

Thanks for the rolls. I tried several times and it kept buffing on me. 

Oof. Poor Elsara.

Starting Necrotic Damage: 3.

Although able to hear much of the commotion, the yet pervasive darkness he knew would prevent any effort of his to find Elsara within it. Nonetheless, he called out a "fall back to the hallway for now, and someone grab Elsara if you can!" before taking a moment to try and listen out for the creature. Discovering nothing however, he clicks his tongue, puts away his bow, and climbed down intending to head for the doorway himself.

He is slow in moving once on the ground, as much from pain as it was leeriness of tripping over a dying Elsara gets to him. But if he finds her in the process, having already donned his shield, he would try to fruitlessly stand guard over body. If not, he got as far as he could, channeling what little energy had behind and added call to Runt in a "Hang Tough, Ruu. This isn't over yet."

Bonus Action: Rally Maneuver on Runt, granting 9 Temp HP.

Action: Don Shield. (Free Action to put away bow)

(That's weird. I thought I had some healer's kit on hand as I usually buy those first and foremost. But can't see, so I assume I didn't for some inexplicable reason... bad past me.)

Val runs towards the sounds were he heard Elsara fall and grabs her, pulling her out of the room back into the alley. Seeing the terrible wound on her chest he gathers what healing magic can muster form Mother Nature and tries to heal her. 

The spirit of the deer follows them back into the alley. 

(Cure Wounds 2nd lvl on Elsara: 11 

Since we don’t have an actual map I am not sure of the spirit position but consider that maybe she could get 11 of healing from the spirit at the beginning of her turn so she didn’t need to roll that death save. If she uses it, it would have 4 heals left. Remember guys to use it and I will try to keep track of the remaining uses ) 

Val: You can use the spirit to heal Elsara and get her back up.If you want, you choose to not cast Cure Wounds and save the spell slot.

( No, let's heal her up... better safe than sorry and Val likes his little "sister" )