Davis stands ready as Zyl opens the door.

Vesta eyes the cultist with sympathy, but she too passes them by and stands toward the back of the party, waiting for Zyl to open the next door. She braces herself for whatever may be on the other side.

Zyl opens the door and reveals a vast chamber (40'x85')!  You stand upon a raised gallery at the west end, with another raised section at the east side. Six chanting figures ring the east gallery, their hands raised toward a spherical cage hanging from the 30-foot-tall ceiling. The ritual’s leader chants from the west balcony, surrounded by hunched, one-eyed creatures with knobby hides. A terrified elf struggles in the dangling cage. (Map!)

Roll for Initiative!
Leader: 7
Nothics: 4
Cultists: 15

Katarina Initiative: 10

Her magical rage used up, Katarina is suddenly much less sure of herself and thus more hesitant to act than usual...

Zyl shrieks at the sight all the things.

(Initiative 9, rolled in long)

Davis intiative: 18

(Vanizi Initiative: 12)

Vesta Initiative - 11

Round 1:
 - Davis
 - The cultists at the far eastern side of the room continue their intricate chanting wild gesticulations, hardly even noticing you... (2)
 - Vanizi
 - Vesta
 - Katarina
 - Zylynne
 - Leader
 - Nothics
 - Celeste(?)

Davis will bonus action Misty Step just south of the yellow Nothic, next to the Caster, and then attack the caster!

20 dmg 15

21 dmg 11

If this caster is undead or a fiend, add this damage: atk 1: 1 atk 2: 3

Assuming she has line of sight from where she stands, Vanizi will cast Eldritch Blast at the yellow Nothic. 
(Spell Attack: 20, Damage: 10 force)
(Spell Attack: 29, Damage: 9 force)

Zyl will run toward the Red Nothic and start swinging! "Don't...you look WEIRD!!!"

Attack 1: 17 to hit; 11 magical slashing

Attack 2: 27 to hit; 18 magical slashing

Bonus action: none

(Celeste Initiative: 15)

Vesta moves forward until she has line of sight on the cultists. She is glad none of her companions have run up to the cultists yet for she points her staff toward them and recites the incantation in a clear, confident voice. Suddenly a fireball erupts in the middle of the cultists!

Dex 19 to save (4th level spell) or take

29 fire damage or half on a save

Also, her Elemental Affinity to fire adds an extra 5 points of damage to the spell.

Also, she will use 1 Sorcery Point to re-roll 5 of those damage dice for the Fireball (re-rolling the 1s, the 2 and one of the 4s) using her Empowered Spell Metamagic.

16

So total damage would be 41 points of fire damage to each cultist affected unless they save.

Round 1:
 - Davis seems to fade into mist before suddenly reappearing beside the leader! The paladin then strikes at the woman twice.
 - Celeste ...
 - The cultists at the far eastern side of the room continue their intricate chanting wild gesticulations, hardly even noticing you... (6)
 - Vanizi sends twin beams of crackling energy towards the yellow nothic, smacking the weird creature with both attacks.
 - Vesta rolls in and unleashes the heavy artillery on the cultists conducting the ritual! (Saves in Spoiler below to save space) 
 - Katarina ...
 - Zylynne  runs toward the Red Nothic and starts swinging, connecting with both attacks!
 - Leader
 - Nothics

Map

Katarina will go ahead and rush in beside Zylynne and try to help her take out the Red Nothic (using Feline Agility if necessary to cover the distance).

Crystal Longsword vs. Red Nothic
Attack: 29; Damage: 12 (magical slashing) + 1 (radiant)
Attack: 11; not bothering to roll that damage.

As Vesta's fireball decimates the cultists and disrupts their ritual casting, a riot of silvery-purple energy fills the room. You feel a sense of space tearing open—then you’re falling, and everything goes dark.

Around the world and across the planes, you perceive innumerable cults of Vecna. They snatch away people and strip their secrets in rituals like the one you stopped. Behind them, the withered form of Vecna gathers the secrets like threads, adding them to a glowing sphere of hidden knowledge in some impossibly distant place. The vision fades into darkness, leaving only Vecna’s glaring left eye.

Your vision resolves slowly, revealing that each of you are lying in an open coffin, jumbled together at the bottom of a 10' deep pit. You hear thumping coming from one of the coffins, sealed and stuck halfway in the dirt. You also hear the hungry shouts and slavering of approaching ghouls above you...

The coffin-filled bottom of the grave pit is difficult terrain. The muddy, sloping sides require a successful DC 10 Athletics check to ascend. (We'll hold here until after Christmas - although you are free to post thoughts...Merry Christmas =)