Razz will use Minor Illusion to create a creeping echo like voice of Fiona saying, "You know you can do it, don't stop now"

Deception Check: 17

The woman stares at the ground before she glares up at you. "..... You lying viper. There is no spell, is there? You just wanted to see us kill each other. Strahd was right. You are just a bloodthirsty assassin. Well to the hells with it! I'm not gonna try to kill two more of us. Not when they could do the same to me. I'm not gonna chase some dreams of a devil just for your entertainment. And if that is you, Fiona, you and your 'husband' can go straight to the hells!" The other two have similarly determined looks. 

Initiative: Razz, Kess, female cultist, Ada, young cultist, Chipper, Garrick, balding cultist

Razz's turn.

Razz will take up as high of a advantage point as he can from the cultists and will cast Flaming Sphere and try to have it land with just the cultists within range.

Each will take 10 fire damage or 5 if they make a DEX 15 save.

"You lot wanted to play with fire—so tell me, how’s this feel?"

Saves: 7, nat 20, 14

The cultists flinch in pain, but it looks more like irritation to you. 

Kess' turn.

"You seemed eager to do it once you had an excuse." Kessintra aims an arrow at the woman.

Bonus Action: Steady Aim

Action: longbow to hit w Advantage 25, damage 8 + 8 sneak attack= 16 piercing 

The woman clutches her shoulder after Kess' arrow hits it. "Ah! Curse you!" 

She casts hold person targeting Kess (DC 11).

Ada's turn.

Kessintra Wis Save: 19

Ada could not believe how quickly the cultists turned on each other. She had to give credit to Kess for such a clever ruse, although she didn't necessarily condone it. The paladin would have preferred a more peaceful solution, but the cultists were too far gone. She stepped forward to engage the female cultist, aiming her halberd at the woman's chest.

Halberd - Attack: 18 Damage: 6

Halberd End - Attack: 12 Damage: 5

The woman dodges the first attack. She grins, until she gets smacked in the nose. She's clearly on her last legs.

The other cultists don't even try to help her. The younger one casts hold person (DC 11) on Kess. 

Chipper's turn.

Kessintra Wis Save: 10

Kessintra lets out a hiss of annoyance as the young man's spell paralyzes her.

With a quick chant, Chipper sends a psychic spike into the younger cultist's mind.

Mind Sliver @ younger cultist
Damage: 3 psychic
INT Save DC 15 and -1d4 to the next save (which should apply to the concentration check I believe)

Int save: 2  Concentration: 18-1=17, success

Garrick's turn

Garrick fires his crossbow twice but misses both times (6 & 4)

The balding man, seeing an opportunity, runs up to Kess. He puts his hand on her shoulder and casts inflict wounds at 2nd level.

Attack roll w/ advantage: 7

The magic fails to take hold on her. Razz's turn.

Kess' eyes light up in amusement as the man's spell fails even with all the odds on his side.

Razz flits across the room, weaving through the chaos with effortless grace. As he moves into position, his hand crackles with golden energy, the light of the Morninglord swelling around him. With a playful smirk, he lifts his palm toward the cultists.

"Fire to burn away your sins, light to make sure you see them first. Hope it’s not too bright for you."

As the divine radiance bursts outward, washing over the enemies, Razz wastes no time—darting out of range once more before giving a flick of his wrist. The Flaming Sphere rolls forward at his command, the fire now hungrily licking toward its final mark. Watching it blaze, he grins, wings fluttering with satisfaction.

"And this little light of mine? Oh yeah… I’m definitely going to let it shine."

Move into position

Action: Word of Radiance for 5 points of Radiant damage

Move out of the way of the sphere

Bonus Action: move sphere into place to hit as many of the remaining cultists as I can without hitting party members. They'll take 7 damage, or 3 on successful DEX 15 save

(I'm assuming I have enough movement for all of that)

Action: Word of Radiance

Con save: 13

The female cultist falls back at the burst of light. When it clears you see her laying on the ground, dead.

Dex saves: 21, 7

The balding man seems to take it harder than the younger one.

Concentration save: dirty 20

Since Kess is paralyzed, she can't take her turn. Ada's turn.

I just realized that Kess gets another save at the end of her turn.

Kess' Wis Save: 8, fail

Ada pivots her attention to the younger male cultist as the woman falls.

Halberd - 12 to hit, 4 damage

Halberd End.- Nat 20 for 9 damage