Kaylyn calls to Pensive in her mind to have him come to her.   He'll fly towards her as quick as he can (dash) plunging into the darkness and using his blindsight to find her. 

Meanwhile Kaylyn fires a blast of magic towards where she last knew Gorlak to be.

With disadvantage

Attack: 15 Damage: 7

"I hate you all," Kastix whispers as he attempts to move out of the darkness while trying to be on guard, ready to dodge as best he can at the situation. 

 Round 1 cont...

 A blast of magic from Kaylyn bursts out of the darkness to strike Gorlak, who grunts in pain and breaks his gaze with Sekkar for just a moment. The shadow demon glides into the darkness, presumably to attack either Kastix or Kaylyn. Both can sense its dread presence, but neither feel any sense of injury. Pensive arrives shortly after, landing on Kaylyn's shoulder and projecting a sense of fear at the new danger. Kastix takes the opportunity to scoot out of the darkness, making sure to avoid any further attacks by the demon.

Rhayna: I should have asked you to go as well.
Kastix: Do you wish to move anywhere in particular?
Kaylyn: You'd better roll initiative for Pensive too.
Everyone else... I might do each round in segments, so you can respond to the situation in initiative order. I'll call for your actions...

away from the entrance if kastix can

Pensive initiative 14

Rhayna will cast Guiding Bolt on the Shadow Demon, then move closer to the action.

Attack: 22 Damage: 9

(Don’t want to mess post above.)

Rhayna will try to move within range to cast [Tooltip Not Found] on the Shadow Demon.

 Round 2

Rhayna launches a shining bolt of moonlight into the darkness, unsure of its effect but hearing a loud fiendish hiss.

Gorlak continues to stare down Sekkar, seemingly unwilling to charge into the Dawnbringer's bright aura. He again lifts his hand, pointing at the ranger, and uses some kind of magic.

Sekkar next

((Post 2625? https://www.dndbeyond.com/forums/d-d-beyond-general/play-by-post/47787-zorgs-lost-souls?comment=2625 ))

If i understand rules correctly that combo is supposed to be possible: with "Dread Ambusher" he has 40ft speed and can reach duergars. So, he can use BA: Zephyr Strike, moves to Gorlak, attacks as planned (plus 1 on the first attack for Zephyr Strike) and returns without provoking AO to a couple tiles lower, still bottleneck-ing the entrance.

Would it work?

(I have a Horizon Walker Ranger, the DM just assumes I start every encounter with Zephyr Strike. Love that spell.)

Round 2 continues   (also updated Round 1)

Sekkar's first attack on Gorlak was so lightning fast that no-one saw him move (even the DM). But Gorlak felt it, the pain from the sword only slightly worse than that from Dawnbringer's light. Scowling at the ranger, Gorlak points in his direction with some kind of spell, runs toward Sekkar and then... vanishes! Only a silvery mist remains, although the sound of footsteps can be heard on the footbridge that spans the two mesas.

Now Sekkar's turn for Round 2. Also post another attack roll for Round 1, since Zephyr gave Sekkar advantage.

Round 1 another attempt: Attack 1: Attack: 27 Damage: 13

Round 2 There was no more point in guarding the entrance and Sekkar moved towards archers: BO: Hunter's Mark handaxe: Attack: 23 Damage: 12 + 6

 ROUND 2 continuing

Rhayna has elected to move, which places her in range of Gorlak's attack, so I've changed his turn again to include an attack on Rhayna. Both are in the darkness, so he rolls with disadvantage.

If Sekkar is using a handaxe, he will need to either drop his shield or his sword. If neither is acceptable, he can apply his attack roll to Dawnbringer. It makes a difference, because if Dawnbringer is not active, the duergar aren't affected by the sunlight.

Round 2 cont...

(DuergarA disadvantage roll: 21)

The two duergar at the entrance respond to Sekkar's charge, one dropping further back and continuing to fire, and the other dropping the crossbow and summoning the now-familiar energy blade. The crossbow bolt again flies wide of the cunning ranger, but the blade strikes true, sending a shock to the lizardman's body rather than any actual wounds.

(BlueBayou: I'm going to consider it unlikely that Sekkar dropped or stowed Dawnbringer. He had movement enough to get into melee with the duergar, and the advantage of the sunlight is just too great. If there was a legit reason to switch to handaxe, let me know... but it would be necessary to either drop the shield or the sword, or stow the sword.)

Pax/Zeno/Kaylyn next

(Has Pensive made his way to Kaylyn yet?  If she called to him last turn he would be before her in initiative and would be heading toward her as fast as he can.)

Pax, seeing the lizard man battling three foes by himself, takes a running leap, and leaps off the edge cliff passing the 2nd level entirely and flying down to the ramp below.  She seems to fall in slow motion however (Feather Fall cast in reaction to leaping off the edge) and lands on her feet, continuing to rush into battle at Sekkar's side. (move, action: Dash, total movement 60 feet). As she rushes across the open ground she draws her blade. 

Quote from Paladin54 >>

(Has Pensive made his way to Kaylyn yet?  If she called to him last turn he would be before her in initiative and would be heading toward her as fast as he can.)

 Yes, 60' is pretty fast.

Zeno grimaces as the shadow demon disappears into the darkness.  He turns to line up with Gorlak, who also disappears, then with the Duergar who he decides are out of range. Frustrated, he turns back to the shadow demon and fires into the darkness where he last saw it.

Attack: 15 Damage: 11  (Assuming advantage  from Hide cancel disadvantage from Darkness. If not here’s another Attack roll: 22 )

Hearing the footsteps on the bridge above, he shows the crossbow, drawing the rapier, and heads in the darkness, up the stairs, to the egg platform above.

The darkness settles around Kaylyn and she is blinded by its depth.  A moment later though a touch of reassurance is felt as Pensive lands on her shoulders and he curls up around her neck.  Using his blindsight she looks around for the demon she is sure has moved into the darkness.

She prepares her magic and sends forth burst of eldritch energy at the demon

Attack: 12 Damage: 14

Round 2 continued 

Pax's running leap and graceful glide 20' to the entrance ramp almost looks like the flight of a bird (of which none have been seen in the Underdark), and brings her almost into striking range of the duergar fighting Sekkar. Zeno pulls a shot off and then slips quietly into the darkness to seek another foe, while Kaylyn's blind shots still manage to find her target, eliciting more angry hissing from the unseen demon as she senses it withdrawing from her reach. Kastix curses the gods of fatalistic irony as he somewhat unsurprisingly sees the demon emerge from the darkness and glide toward him. It's claws reach out and swipe at him, causing no wounds but creating a sharp stabbing pain in his mind that makes him think horrible thoughts of anger and violence. The demon then slips back into the darkness as silently as it emerged.

- Kastix takes 8 pyschic damage.
- Zeno's dice rolls aren't showing. Maybe try to edit the post by adding a space anywhere, and it might fix it.
- Kastix can make an opportunity attack against the Shadow Demon
- Kaylyn can also make an opportunity attack against Shadow Demon but with disadvantage
- Kastix and Rhayna can take their turns. Both will need to make Perception checks of 14 or higher if they wish to target the demon.