Initiative: (I'll use the highest bonus within each side, so that way I don't have to type in the command for 15 different characters and then comb through them looking for the highest)

(well...I think we can safely say the parties going first. Man, a negative initiative score. Damn. Never thought I'd see that)

First up...

the Party! Time to play by the rules and slay these fools!

Khessa tries to transform the space of the door into a sort of bottleneck, positioning herself for example just before the threshold, so that only (depending on the size of the opening) one or two enemies at a time can arrive in melee with her - and possibly no enemy can go past her to attack whoever wants to fight from range. I imagine some other melee fighters can take position next to her if they want (there should be more space before the threshold than at the threshold).

The blonde arcane warrior, once she is in position, while awaiting her enemies with the Greatsword in hand, momentarily detaches her right hand from the hilt, in an elegant gesture accompanied by a short formula, and a mote of fire starts from his hand aimed at the nearest self-moving corpse.

The tall adventurer then begins using both hands again to wield her long blade.

Khessa's Fire Bolt Attack: 9 Damage: 8 (fire)

Khessa's bolt of fire streaks forward, and despite her aim being thrown slightly off by the jockeying for sight into the next room, it still lands, blasting one of the foul creatures backwards and singeing in its abdomen.

The tunnel is wide enough that two or three people could stand side to side (including Khessa) and be within 5ft of the zombies.

Everyone besides Khessa, you know what to do!

Without hesitation, Kit steps in to close the gap to Khessa's immediate right. Her sword is raised, readied to slash the first zombie that shambles into her reach.

(Would you mind rolling that attack now, just so we've already done it when the zombies do reach the party?)

(Not at all! Longsword Attack: 18 Damage: 6)

Erven pulls out his longbow, seeing the Zombies and due to his Dread Ambusher skill, he rapidly fires off two arrows his first round, gritting his teeth as he sees his most hated enemies.  He actually growls under his breath, sizing up his distance to the targets.  He’s near the back of the pack since everyone rushed forward to open the door, he starts to move forward as he shoots.  

Longbow shot 1 : 10 to hit, for 8 points of piercing damage.  

Longbow shot 2 : 26 to hit, for 7 points of piercing damage, and an additional 4 points of damage due to his Dread Ambusher skill.  

Ketangu moves in and stabs at the nearest zombie with his spear. He has 50ft walking speed. 

Spear attack

24 to hit 6 damage - rolled in game log

Unarmed Strike as a bonus action

21 to hit 5 damage - rolled in game log

He then retreats back to 10 ft behind Kit. He has the Mobile Feat and does not provoke any opportunity attacks. 

End of Turn

Tabitha will climb on Lance, and direct him to take Khessa's left. She holds her mace in one hand, her other hand free (lacking a shield. Subtle hint :p) ready to hit the first undead that gets near her. She also commands Lance to attack when one gets in range.

 When the first zombie within 5ft of Lance attacks, it will be with disadvantage due to the deflect attack reaction.

Still standing back in the corridor behind his more brave companions, the young dark-haired man groans as he spots the shambling dead. The undead were such a tedious sort with absolutely no sense of humour. He considers how the cultist would have been able to pass the zombies, maybe there had been a way to avoid a fight, but now the battle was in full swing and he decides to follow suit, placing his hex on the closest clumsy corpse and then blasting it with a greenish beam of eldritch force from his outstretched hand.

Eldritch Blast: 19 Force: 13 Necrotic: 2

He then steps back further, gicing place for the markswoman Kevori to use her bow.

Seeing all his companions spring into action at the first sight of the zombies, Bahrumuhd takes a moment to appreciate their quick thinking and reflexes. Definitely a useful to keep around if you were expecting to get into fights a lot. 

Deciding that he was going to use his innate breath weapon that all dragons posses, Bahrumuhd steps a bit off to the side and waits for them to cluster together a bit more. Might as well hit as many of them as he can. 

(Holding a breath attack until after the zombies move forward and bunch up into a group. Of course, he wants to hit as much zombies as he can, so he'll aim his 15ft cone to do such. Anything in the cone will take 7 lightning damage if they fail a DC 13 dexterity saving throw, taking half damage on a successful save) 

As Khessa, Tabitha, and Mud hold the front line, Erven and Ketangu spring into action. Erven, his shadowy form concealed in the dark, launches two piercing arrows which strike one of the shambling corpses (Z7), first in the leg and then the shoulder. Vile liquids squirt from it's wounds. Ketangu draws his trusty spear, becoming a whirlwind as he oozes forward, tactically dodging into a position where is now flanking the zombies as they lurch towards the bulk of the group. After his onslaught is done (against Z5), he turns to a puddle, retreating behind Kit, who swings her sword as the first zombie (Z1) approaches. Sadly, despite the slow reflexes and soft skin of the creatures making them easy to hit, the oozing of Ketangu beneath her feet surprises her, throwing her off balance and causing her strike to swing wide. Tabitha swings her mace at an approaching zombie (Z2), and it connects with a satisfying crunch, while Jack sends forth a bolt of energy, which twists through the air between combatants, until it strikes another of the foul creatures (Z8). To finish off the furious display, Mud reaches deep inside his dragon-touched soul, and calls forward a crackling cone of silver lightning, which sparks all over the zombies (Z1-6), charring flesh.

DEX save for Z1 & Z2: 2; DEX save for Z3 & Z4: 12; DEX save for Z5 & Z6: 13

Z7 & Z8 are too far away to be hit by the Breath Weapon. The one that Khessa hit was Z3.

Kevori's Turn!

Kevori steps forward, filling the space that Jack was in, and fires her bow at one of the lurching zombies (Z6). Her arrow flies through the air, gleaming in the dim torchlight

Bow Attack VS. Z6: 14 to hit. Hit: 5 Piercing damage

Zombie's Turn!

The foul creatures lunge, terrible claws and gnashing teeth assaulting you along with a tidal wave of noxious stench. Everything down here smells bad. The zombies crowd forward, pressing into each other, making it harder for them to attack, but allowing more to reach the party

Z1's Slam Attack Vs. Khessa: 13 to hit. Hit: 6 bludgeoning damage

Z2's Slam Attack Vs. Khessa: 10 to hit. Hit: 5 bludgeoning damage

Z3's Slam Attack Vs. Tabitha: 12 to hit. Hit: 7 bludgeoning damage

Z4's Slam Attack Vs. Tabitha: 8 to hit. Hit: 2 bludgeoning damage

Z5's Slam Attack Vs. Mud: 7 to hit. Hit: 7 bludgeoning damage

Z6's Slam Attack Vs. Mud: 13 to hit. Hit: 5 bludgeoning damage

Z7 & Z8 cannot reach the party, since they are in the back

Parties Turn!

Still standing back in the corridor, the young dark-haired man briefly steps closer to find a clear shot at the hexed clumsy corpse in the back (Z8), blasting it with a greenish beam of eldritch force from his outstretched hand.

Eldritch Blast: 15 Force: 9 Necrotic: 5

He then again steps back further, giving place for the markswoman Kevori to use her bow.

Lance manages to butt Z1 out of the way as it lurches to attack, the movement bringing Tabitha easily out of the way of Z3 and Z4.

She then takes a swing at Z3, commanding Lance to do the same. Lance unfortunately fails to make purchase in his attempt to make sure he doesn't catch Khessa with the force rend.

Ketangu oozes past his companions and attacks Z5 

Spear

17 to hit 10 damage - rolled in game log

Unarmed Strike as bonus action

19 to hit 8 damage - rolled in game log

Then he retreats by oozing back to 10 ft behind Kit. 

End of turn. 

Kit curses in Elvish under her breath as she once again misses her foe. Perhaps later she'll tell herself it was the sweat stinging her eyes and causing the blade to twist in her grip. It certainly couldn't be fear or ineptitude. For now, however, she merely grits her teeth and redoubles her efforts in an attempt to prove her worth...to her companions and herself.

(Longsword at Z1, Attack: 18 Damage: 7)

Khessa had hoped to organize a defense line so that the party's front-liners would outnumber (or at least avoid being outnumbered by) the attackers, but she evidently miscalculated either where to place the defense line or the enemies' ability to crowd into confined spaces.

The blonde arcane warrior is however satisfied with how the battle is going... the disturbing undead have not managed to claim any victims and, contained as they are at the moment, they are suffering under a storm of steel and magic... They should begin to collapse soon!

'The one I burned, for example!' the tall adventuress fixes her blue eyes for a moment on Z3 'And who was subsequently hit in full by Mud's Dragon Breath! Maybe a well-aimed blow will be enough to finally make him Rest In Peace...'

And the next moment the heavy Greatsword is already descending towards the decomposed flesh...

[[ OOC: Khessa attacks Z3 with her Greatsword, hoping to finish it. If Z3 is dead already, the nearest zombie will do ]]