Gorin activates his protector cannon to give all allies within10ft of Zim 11 temp hp.

If he can find a piece of object between 1 and 5 pounds around him he'll cast catapult; DC14 dex save or take 12 dmg.
If not another fire bolt: to hit 17 for 3dmg

Marasatra presses two fingers to the amulet at her throat, channeling a quick ward into Arkon; a faint lattice of runic light settles over the goliath’s skin to blunt incoming steel (Resistance vs. Piercing damage). Without breaking focus she flicks her other hand toward the bound figure in the corner, voice dropping to a terse incantation—soft, restorative syllables that send a pulse of healing energy across the chamber (Healing Word for 7 HP at the chained and masked figure hoping to enable it to join the fight or get out).“Strength returns—use it now! Tear free and get clear, or fight for revenge!” Ekko is already halfway to the captive’s position, and soon ready to tug or gnaw the bonds in an effort to help the prisoner, too.

Arkon frowns, confused by Vaaz's excitement. "Never seen someone so happy to lose." He glances over, relieved to see Zim stand. "Are you better now? What happened?"

Quote from EmileTherese >>

Arkon frowns, confused by Vaaz's excitement. "Never seen someone so happy to lose." He glances over, relieved to see Zim stand. "Are you better now? What happened?"

Zim snarls, "I was compelled to kneel, to a death god. Yuck!"

Vaaz grins, "Who doesn't love a good fight!" He gives a manic laugh as body tenses up and hardens . His skull glowers at Zim.

Zim WIS save, DC 14, on fail stunned until the end of your next turn.

He stabs at both Arkon and Zim.

Attack at Arkon: 17 Damage: 8
Attack at Zim: 22 Damage: 9

( Advantage roll incase Zim fails save: 25 & Vul+Res cancel each other out so normal damage on Arkon but 2x on Zim )

The masked cultist gives a sharp inhale as Mara's spell invigorates them. "The hells...Damn it! Free me so I can kill the bastard!" They shout.

Quote from Virst>>

Vaaz grins, "Who doesn't love a good fight!" He gives a manic laugh as body tenses up and hardens . His skull glowers at Zim.

Zim WIS save, DC 14, on fail stunned until the end of your next turn.

Zim attempts to save against stunned, Wisdom Save DC 14 vs 18,

Vulnerable to the 10 damage means that Zim is taking 20. With his 11 temps, that's just 9 real damage, unless he can parry.

If stunned, he can't take reactions. But if he saves then Zim will use a reaction to parry (1d8+4 damage reduced) 11,

Zim takes the 20, minus 5 from parry, minus 11 from temps, so his new hit point total is 50-4=45

Quote from Virst >>

 

( Advantage roll incase Zim fails save: 13 & Vul+Res cancel each other out so normal damage on Arkon but 2x on Zim ) 

The spell Resistance doesn't grant resistance but reduces damage by 1d4 (which is reduced before vulnerability or resistance applies), for brevity here's the roll: 4, which means Arkon takes [roll](8-[roll:-1])*2[roll] damage against his 11 temporary hit points (unless he has more ways to reduce the incoming damage).

Quote from NathanAscher >>

Zim takes the 20, minus 5 from parry, minus 11 from temps, so his new hit point total is 50-4=45

Damage reduction applies before resistance or vulnerability, this means from the 10 damage he parries 5, then the 5 remaining are doubled to 10 and only reduce his temporary hit points to 1. Zim didn't take real damage from the attack thanks to his parrying and for being protected by the Gorin's shield. 

Not going to edit and spoil the great roll... but my roll code didn't work due to a missing slash. With this Arkon would only take 8 damage which is fully absorbed by the temporary hit points from Gorin's shield. (PHB section about Damage for the source of order of damage reduction/resistance/vulnerability)

Ekko (Mara's bat) swoops down, tiny fangs worrying the rope fibers at the captive’s wrists (helping with an Athletics check to break free). Strands begin to fray under the rapid nibbling (assuming that they are bound by rope). “Pull—now!” Marasatra urges, eyes still on Vaaz. “The bat can’t cut it alone—break the last threads and arm yourself!”

Arkon notices the change in Vaaz's body and wonders if it is similar to his own Stone's Endurance...Either way, he just has to do more damage than the skull-faced man can withstand. "Keep your creepy stares to yourself!"

Action: Greatsword to hit 15, damage 13

The tiny invisible imp again distract Vaaz before taking cover.

The young red-haired patriar again unleashes a hellishly red beam of eldritch crackling force towards the hopefully distracted Vaaz before dodging back into cover.

Eldritch Blast: 11  Another attack roll if helped by Mal's distraction: 13 Force: 10 Necrotic: 5

(The cultist will act at the end of the round.)

Arkon's blade fails to cut into Vaaz, his body seemingly hardened. Valeria's eldritch blast is just off by a bit as it only glances Vaaz.

(Gorin and Zim are up, will bot Zim.)

Bot Zim

Attack: 10 - Fail to hit
Action Surge: Attack: 21, Damage: 10, Sap applied to Vaaz

First of all, Gorin activates his protector cannon again to give everyone withtin 10ft of Zim a new 11 Temp hp.

After which he throws another fire bolt at Vaaz:

To hit 12 for 6dmg.

Marasatra wades in beside Arkon (stepping into melee), shield held high and staff angled to jab. “Eyes on me, skull-face,” she challenges, crowding Vaaz’s flank to open his guard. “Arkon—now!”

(she is effectively helping Arkon to land a blow.)

Arkon swings again, taking advantage of the distraction.

Action: Greatsword to hit nat 20+7, damage 22

Initiative Order -

The cultist breaks free of his bonds and darts into the room where Valeria and the others were, heading towards the boxes. "There were healing potions last I checked here. I need a few after that bastard knocked me out." They say as they open a crate and start to sift through the packing hay.

"Oi! Come back! You're just going to leave your comrade here? Coward!" Vaaz laughs as he sees the cultist run off into the room. "Well...atleast you all didn't leave me alone. Care to see what you're afraid of?" He looks to Mara, his stunning gaze focused upon her. His knife flicks between Zim and Arkon once more.

Stunning gaze at Mara, WIS Save DC 14
Attack: 16 Damage: 11 at Zim
Attack: 13 Damage: 10 at Arkon

Quote from EmileTherese >>

Arkon swings again, taking advantage of the distraction.

Action: Greatsword to hit nat 20+7, damage 22

 Vaaz takes the blow head on, but...it seems to leave no scratch upon him. "That was a bloody good swing...do it again!"

Damage done:

• 70/?? to Vaaz

Mara’s eyes widen as Arkon’s cleaving stroke skitters off Vaaz’s body. “His hide’s warded—check the statues or some spell focus. I believe we’re missing the anchor!” she calls—but the skull-mask snaps toward her, hollow sockets blazing. The malice behind the gaze hits like a hammer; her warning chokes off, and she stiffens mid-stride, shield sagging as every muscle locks in place (Wis save 10, I suppose she's stunned) but only for a split second as her iron determination finally gets the upper hand! (Using Heroic Inspiration for a re-roll: Wis save 18!)

Meanwhile, Ekko wheels overhead with a shrill screech, feinting straight for Vaaz’s eye sockets to throw the priest off-balance—but at the last instant the little bat veers away, tucking behind Mara to cling to her cloak while she holds Vaaz's ugly gaze heroically.

Mara Save: Mara's Wis save 10 vs DC 14, but Heroic Inspiration saved her with an 18! 
Ekko's Move: 30ft back to behind Mara (c2)
Ekko's Action: help Sarris to land his shot

After watching as Arkon's exceptional head strike skip aside, Sarris groans and thinks to himself,  'I bet it has to be magical weapons.  Dammit to the Void.'  

He yells to his companions, "Try magically enhanced attacks!  Something has to get through to Old Bonehead."