Davis Dex Save: 19.

He adds a curse and some burning to his original action, but will do the same, head down to the red cultist and attack, trying to make his way to the leader.

Katarina Dex save: 11 (even with advantage... I've noticed a lot of doubles when rolling in the game log recently), plus 5 since it looks like she's within Davis' aura, for 16 total.

Her action does not change at all... if anything, she's now MORE eager to reduce the cultist leader to a smear on the ancient stones.

Round 1 (con't)!
Zyllynne flies down from the landing, over the table, to land between the blue and yellow cultists. She rages about the unfairness of it all, growing in size and lighting her axe on fire! Then with two mighty blows, she drops the blue cult fanatic to his knees, barely holding on for dear life. (1 hit point left!!!) The man just flops on the ground, trying to keep his arm attached to his torso.
Leader points at the group, and a small orb of liquid magma forms. It zooms towards the party, growing in size as it flies at you! Lucky for Zyl that she just flew away - unfortunately, everybody else is caught in the fireballs blast. (Make a DC14 Dex saving throw, taking 35 fire damage if you fail, and half if you save...)
Celeste (-35/17) summons a beast of the land to maul the spellcaster, then shoots him with a luminous arrow. All attacks hit in retaliation for the scorching the party just received! 
Katarina (-17) rages at the burning in her flesh and somehow blinks out of sight only to reappear next to the leader! Her reckless attacks are more than the leader can defend against, and with the mauling from Celeste and her beast, the purple leader falls to the ground!
Cultists
 - The Red cultist points at Davis and commands the paladin to [Tooltip Not Found] (Make a DC14 Wis save or spend your turn moving away from the cultist by the fastest available means.)
 - The Green cultist does the same thing, but points her finger at Katarina! (Casting hold person.  Make a DC14 Wis save or be paralyzed for one minute. At the end of each of your turns, you can make another Wisdom saving throw. On a success, the spell ends.)
Davis (-17) moves into the room to attack the red cultist. His first strike goes horribly wide - his grip must have been affected by the fireball! However, the paladin corrects his form and lands a solid blow with a follow-up swing.
Vanizi (-35/17) Stays on the steps just inside the room, hexing the cultist Davis is attacking. She follows up with two blasts or eldritch fire, dropping another cultist!

Round 2 (only Green and Yellow remain!)

Zyl will continue to try and wail on the yellow cultist in her annoyed rage.

Attack 1: 15 to hit, 7 fire +3 (rage bonus) +3 (elemental cleaver) = 13 fire damage

Attack 2: 27 to hit, 13 fire +3 (rage bonus) +5 (elemental cleaver) = 21 fire damage

Bonus action (if the DM will allow it): Mighty Impel

If the cultist is still alive, make a DC 16 Strength saving throw as Zyl attempts to grab and throw them across the room (because she can)

If they're dead, she still wants to do it, but if possible she'd love to throw them AT the green cultist (if not don't worry about this lol)

(Vanizi DEX Save vs Fireball: 9 - yes, includes the +5; removes temp hp, 63/83 hp) 

After taking the full brunt of the fireball, Vanizi moves down the stair to try to get some space from the others. (I guess to the spot in the NE corner) Shifting her Hex to the next cultist (green), she again launches bolts at the target by casting Eldritch Blast. 
(Spell Attack: 14, Damage: 9)
(Spell Attack: 19, Damage: 6)
(If one of the attacks hit, Hex damage: 2 necrotic) 

Round 2

Zyllynne (-0) is in full rage - a little she-hulk destroying all enemies around her! Her mighty swings drop the yellow cultist.
Celeste (-35/17)  
Katarina (-17) 
Cultists
 - The Green cultist 
Davis (-17) 
Vanizi (-20) (action noted - will add it after cultist turn)

Katarina Wis save: 20. The feline is apparently too angry to be stopped by some stupid spell!

(Waiting on Celeste's and Katarina's actions...)

Katarina:

The feline hisses in frustration, her fur still smoldering from the fireball.

"Katarina's hair is RUINED!" she snarls at the nearest remaining cultist. "It could take her all day to fix this mess!"

Bonus action: Teleport in front of the GREEN cultist

Action: Two reckless attacks against said green cultist.

Longsword vs. Green:
Attack: 22 (10, 22); Damage: 7 (magical slashing) + 5 (radiant)
Attack: 19 (17, 19); Damage: 12 (magical slashing) + 2 (radiant)

Dex ST vs DC14: 22
(HP -17)

Quick on her feet, as if a familiar to the wild nature of the elements, Celeste dances out of most of the flames. And as she does, she speaks in druidic to the fire as if it were an old friend, absorbing some of the heat into her luminescence.

Reaction: Absorb Elements
(HP -8)

Turning her attention to the green cultist, Celeste releases her luminous arrow their way.

BA Luminous Arrow Attack: 18 Damage: 8

Should she be able to close the gap, the druid would take a swipe at the cultist with her moonsickle.

Action Melee Attack: 21 Damage: 5 + 3 fire damage
(If not, she'll Frostbite Con DC19 Damage: 6)

Round 2 (updated map!)

Zyllynne (-0) is in full rage - a little she-hulk destroying all enemies around her! Her mighty swings drop the yellow cultist.
Celeste (-8) dodges the fireball and even manages to absorb some of its energy before focusing on the last cultist with a luminous arrow and then some frost. (DC19 Con save: 21) 
Katarina (-17) rages some more, then moves to attack the last cultist, dropping the poor soul (green down).
Two new Cult mages emerge from the door in the western wall, ready for a fight with the party! They both make the same intricate hand movements and speak words of power, holding their hands in front of them as blasts of frigid air hits everyone. (Casting cone of cold to cover the room. All must make a DC14 Con save, suffering 27 cold damage on a fail or half as much damage on a success.)
Davis (-17) 
Vanizi (-20) (will let you re-write your action based on the new threats...)

(Con save: 15, rolled in log. Zyl is at 91/115)

(Sorry, want to clarify - 2 Cone of Cold spells were cast? But everyone is hit by only one? I ask because it's made to sound like 1 spell)

(Reserving Save and possible Reaction depending on answer to above question.)

Vanizi moves down the stair, trying to take a position a bit away from her companions to try and threat area effects.  She moves her Hex and casts Eldritch Blast at one of the new cultists. 

(OOC- Same as original move but changing target)

(Yes, 2 mages, each casting a Cone of Cold, and as your benevolent DM I aimed them so each character is only hit by one =)

Davis will move directly west, north of the Pink Mage.

Bonus: Hunter's Mark on the mage.

attack Holy Avenger Longsword: 19 slash: 19 (w/ Hunters Mark)

attack Holy Avenger Longsword: 15 slash: 11 (w/ Hunters Mark)

Katarina's Con save: 20 (12, 20. Katarina has advantage on saves against spells and other magical effects from her spellguard shield)

I'll use Katarina's reaction to use Unstable Backlash and roll again on the Wild Surge table. 4:
One of Katarina's weapons becomes infused with magic and gains the light + thrown properties. It also deals force damage. Overall, not particularly useful considering her crystal longsword already deals magical damage and bonus radiant damage... I guess one of her handaxes gets infused until the next time something hits her.

Movement: to a square adjacent to the BLUE mage.

Action: Attack twice with her crystal longsword. She attacks recklessly.

Longsword vs. BLUE cultist:
Attack: 17 (10, 17); Damage: 9 (magical slashing) + 2 (radiant)
Attack: 25 (17, 25); Damage: 13 (magical slashing) + 3 (radiant)

Round 2(con't) (updated map!)

Davis moves to the pink mage and gets two strikes in, although the second one just barely! 
Vanizi (-20) 

Round 3
Zyllynne (-0) 
Celeste (-8)  
Katarina (-17) moves to the blue mage and slashes her twice with the crystal longsword
Cult mages 
Davis (-17) 
Vanizi (-20)

"AAAAAAAAAAAAAAHHHHHHHH!!!!!!!!" Zyl stomps over to the new blue cultist as she shakes off the cold from the spell and swings away.

(I'm not sure if this qualifies for flanking since technically Zyl is large size now so rolling straight)

Attack 1: Nat 1! (sad)

Attack 2: Nat 20! (yay!) 18 fire +3 (rage bonus) +4 (elemental cleaver) = 25 fire damage to blue cultist

(CON Save: 9; 36/83 hp)