Kazri does not take what the Tiefling is telling them. She does not speak, but study Avarice to determine what she is saying is true.

Insight: 20

Avarice’s expression darkens, her once-calculated composure unraveling as Krom’s words confirm that Vellynne remains among their ranks. Her lips curl in a grimace, and her eyes narrow, a storm of fury brewing within them. The shift is subtle at first—a tensing of her jaw, a flicker of contempt—but as Rhogar mentions the archdevil she serves, her anger ignites like a spark catching dry kindling.

"So be it, then," she snaps, her voice cold and venomous. "I thought you had some semblance of sense, but clearly, I was mistaken."

Her grip tightens on the staff she wields, knuckles whitening as raw energy courses through it. Her intent is clear—there will be no further discussion, only action.

Initiative - Chill 13, Kazri 9, Krom 7, Rhogar 7, Avarice 23, Cultist 2

A hail of rock-hard ice pounds to the ground in a 20-foot-radius, 40-foot-high cylinder centered around your group.

Each of you must make a DC 14 Dexterity saving throw. 13 bludgeoning damage and 16 cold damage on a failed save, or half as much damage on a successful one.

Hailstones turn the storm's area of effect into difficult terrain until the end of your next turn.

Avarice (?), Chill (73/73), Kazri (100/114), Oli (38/42), Krom (104/104), Rhogar (87/87), Gorrah (27/27), Cultist (?)

Krom's Dex save (Including the +3 from Kazri's Aura of Protection - rolling with advantage due to the Mantle of Spell Resitance): 17
Krom is also within Kazri's Aura of Warding that grants him resistance to damage from spells.

23

Reaction(Rhogar): Cast (3rd level)Counterspell. If 4th level or higher spell, here's the roll: 20. 
If it looked as if Avarice cast a spell, then Rhogar will attempt the above... and hope it isn't counter counterspelled. T.T

If above is still a fail, here's his Dex Save(DC 14): 9. 

Dex save 28

Reaction: Uncanny Dodge to halve any damage taken.

Chill takes Steady Aim (BA) with his crossbow and gets Avarice in his sights and lets a shot go.

Attack: 14 Damage: 23

Dex Save: 25

(Wait for Rhogar's Counterspell result)

Avarice grins as Rhogar tries to counter the spell from her staff. She quickly reacts herself and counters his spell.  The original spell still goes off.

Kazri, I need a dex save for Oli

Chill take 3 bludgeoning and 2 cold (made save, Kazri aura of warding ,uncanny dodge)

Kazri takes 0 bludgeoning and 2 cold (made save, Kazri aura of warding, takes -3 bludgeoning for Heavy Armor Master)

Krom takes 0 bludgeoning and 4 cold (made save, takes half damage for spell resistance, takes -3 bludgeoning for Heavy Armor Master)

Rhogar takes 6 bludgeoning and 8 cold (failed save, Kazri aura of warding)

I hope I got all of that right.  You can't stack spell resistance from the mantle or Kazri's Aura of Warding with the cold resistance

Avarice (?), Chill (68/73), Kazri (98/114), Oli (38/42), Krom (100/104), Rhogar (73/87), Gorrah (27/27), Cultist (?)

(Sorry, keep forgetting our little mascot.) Oli DEX Save: 7

Without hesitation, Kazri charges at Avarice.

Attack1: 14 Damage: 14

Attack2: 19 Damage: 13

Meanwhile, Oli charges at the cultist.

Beak Attack: 22 Damage: 8

Claw Attack: 22 Damage: 12

(If needed DEX Saves to move in the difficult terrain. Kazri: 6. Oli: 20)

Krom chanting an hymn of conjuration advances carefully towards Avarice and the cultist. As the final words of his hymn echo through the air, a score of spectral dwarves materialize around him. Without any hesitation, the spectral dwarves swings their radiant hammers and axes, striking at the tiefling and her companion with masterfully precision.

Movement
2 squares towards Avarice

Action: [spell]Spirit Guardians[spell] at 4th level 30 Radiant Damage DC 15 Wis for half-damage

Rhogar's growl of pain turns into a dark chuckle as he glared at Avarice. "Two can play at that game. But-" The dragonborn taps the ground, sending a shockwave of divine blue energy sweeping through the area nearby, before then leaving Rhogar, Oli, and Krom faintly glowing for only a few moments. Yet well after a feeling of being invigorated remains in their system. With a thought, Gorrah is urged to crawl out from the confines of Rhogar's pack and fly over to sting the wizard most viciously. In that same moment, Rhogar fell back to cover the stairs, fully anticipating a retreat attempt out of the wizard.

Action(Rhogar): Cast (1st level)Empower on Self, Krom, & Oli.

Movement(Rhogar): 10 South East, positioning with intent to block stair passage.

Movement(Gorrah): Exit backpack and fly 5ft directly above Avarice. 

Bonus Action(Rhogar): Command Gorrah to Strike!

Action(Gorrah): Nighty-Night Stinging.
(Nonlethal)Sting - Attack: 11 Damage: 5 (23,6) of magical piercing. If hit, Con Save(DC 16) or become poisoned. If failed by 5 or more, target is knocked unconscious for the duration of the poison(1 hour). Taking damage or another creature using an action to shake them awake breaks unconscious condition. 

Avarice Wis save 16

Cultist Wis save 17

Avarice Con save 4

The battle rages on and your attacks are swift and fruitful.

As Gorrah swoops over Avarice and delivers a sting that knocks Avarice unconscious!

The cultist wields a scythe and slashes at Oli

Attack 7 8 slashing & 6 necrotic

Back to Chill

Avarice (Unconcious), Chill (68/73), Kazri (98/114), Oli (9/42), Krom (100/104), Rhogar (73/87), Gorrah (27/27), Cultist (?)

Chill switches his attention to the cultist as Avarice falls.

Steady Aim

Attack: 24 Damage: 30

Kazri (Oli), Krom and Rhogar (Gorrah) up

Kazri signals for Oli to stand down, then she turns to the cultist. "Your master is down. Do you yield?"

Persuasion/Intimidation: 19

Krom seeing Gorrath taking down Avarice says "Masterfully done!". The dwarf immediately steps to the right side of Oli in order to keep his spectral dwarves from attacking the unconscious tiefling, but within range of the cultist. The dwarves relentlessly assault the cultist with their spectral mauls and axes from every conceivable angle.
Krom unhooks a throwing axe from his belt, balancing it expertly in his hand. "You should listen to the lass. Make one wrong move, and this axe will be the end of you." says with determination.

Movevement: 3 squares to the right in order to keep Avarice out of the Spirit Guardians

Spirit Guardians damage on the Cultist at the beginning of his turn: 17 Radiant damage Wis save DC 15

Reading an action: Krom throws an axe if the Cultist doesn't surrender to Kazri.
Attack: 12 Damage: 6 + 3 of Fire damage

Provided Chill's last attack hadn't put the remaining cultist(sort of) down for good, Rhogar backed off to guard the entrance and made ready to wrestle down any foe daring to make use of it. But as for Gorrah, despite the others suing for peace, she appeared to take the initiative in zipping over to the cultist side and giving him a taste of her stinger to further "encourage" the man to surrender, if not knocking him out as well.

Movement(Rhogar): To square to the southeast to better block stairs. 

Action(Rhogar): Ready to grapple if an enemy that gets within 5ft.

Movement(Gorrah): Into melee with cultist.

Bonus Action(Rhogar): Command Gorrah to Stabith -thee (via telepathic link)!

Action(Gorrah): Nighty-Night Stinging.
(Nonlethal)Sting - Attack: 12 Damage: 7 (12,7) of magical piercing. If hit, Con Save(DC 16) or become poisoned. If failed by 5 or more, target is knocked unconscious for the duration of the poison(1 hour). Taking damage or another creature using an action to shake them awake breaks unconscious condition.