(To make sure you all understand - only Zyllyne and Mulligan are in actual combat. The rest of you are about 25' away in the library - they went alone through the tunnel to see what was going on in the next area. So you'll have to move, then fight through or around your lead members, who basically block the entrance of the tunnel.)

Init order: Cultists (red & blue)-17 :: Shadow creatures (red & blue)-15 :: Mulligan-12 :: Zyl-9 :: Dara-7 :: Celeste-5 ::  Leader-3 :: Vesta-? 

• The two cultists turn and look in your direction, then rush forward with scimitars drawn. One (blue) attacks Zyllyne (Attack: 22 Damage: 5), the other (red) attacks Mulligan on the wall (Attack: 21 Damage: 6).
• The shadows that emerged from the cultists seem to blend into the shadows of the dimly lit cavern (Stealth: Red-13 :: Blue-18). They are nearly impossible to see! (Disadvantage on Perception checks to see them...) One materializes next to Mulligan and attacks the halfling (Attack: 23 Necrotic damage: 10. Mulligan's Str is reduced by 1 on a successful hit!) Zyllyne feels the other shadow flow past her into the tunnel back to where the rest of the party wait!
• Mulligan, suddenly beset by man and shadow, materialize blades of psychic energy and slashes out around him!  His first strike drops the red cultist with a nasty slash across her throat, and the other manages to pierce the shadow and cause it to scream - an unearthly screech that echoes along the passage and instantly alerting the rest of the party that something is definitely wrong!
• Zyllyne feels a shadow flow past her into the tunnel back to where the rest of the party waits and decides it's near-invisibility just won't do. She starts to grow and causes a burst of glowing purple light to coat everything in a 20' cube! (Mulligan - make a DC11 Dex save. Red shadow(15) :: Blue shadow(11) :: Blue cultist(7))
• Dara hears the screech and rushes into the tunnel and sees Mulligan drop one cultist, and then attack a shadow. (Waiting on red shadow save to determine if Dara can see that shadow...) (Blue cultist DC15 Wis(13) His spiritual weapon then follows up the assault on the blue cultist, dropping the half-elf in the gray robe to the ground!
• Celeste
• Leader
• Vesta

(Both cultists are down. There are two shadows, but if they made their Dex save against Zyl's faerie fire, you attack at disadvantage against them since they are hidden in the dim light...Map)

Mulligan Dex Save:  13
     Though can she concentrate on Faerie Fire while raging?

If they succeed she'll rage. If they fail she won't?

Prrt! Celeste runs after the little legs and the others to catch up, while also making sure Vesta was with them and not left behind. She just needs to catch a glimpse of the leader through the group in order to tickle their brain with frostbite. Drawing her bow, she'd also take the chance to release a radiant arrow.

Movement: 25-30ft
Action: Frostbite Con Save DC 13 Damage: 3 and DA on next attack if fail
BA: Luminous Arrow Attack: Nat 20 for 25 if no DA, if DA: 16 Damage: 5 (+7 if crit applies)

Vesta hears a commotion from down the hallway, and she follows Celeste, running toward their friends. As the hallway turns she is able to see Mulligan on the wall being attacked by a shadow! She knows she's got to be careful with her spells or she could hit the halfling too.

Initiative - 5

She decides to keep it simple and hurls a bolt of fire at the shadow.

Fire Bolt - Attack: 12 Damage: 7

(Forgot about advantage so here's another roll.)

Attack: 8 Damage: 4

- Both shadows made their saving throws, so attacks against them are at disadvantage unless your passive perception equals or exceeds their stealth rolls: Red-13 :: Blue-18.
- No, you can't maintain concentration while raging.
- Also, nobody can actually see the leader - he is hiding behind a rock protrusion in the main cavern. 
- The party is stacked up in the tunnel leading to the ritual room, with Zyl and Mulligan in the front fighting a shadow. There is another shadow in the tunnel with you somewhere...)

Round 1 (con't):

• Shadows that emerged from the cultists seem to blend into the shadows of the dimly lit cavern (Stealth: Red-13 :: Blue-18). They are nearly impossible to see! (Disadvantage on Perception checks to see them...) One materializes next to Mulligan and attacks the halfling (Attack: 23 Necrotic damage: 10. Mulligan's Str is reduced by 1 on a successful hit!) Zyllyne feels the other shadow flow past her into the tunnel back to where the rest of the party wait!
• Mulligan, suddenly beset by man and shadow, materialize blades of psychic energy and slashes out around him!  His first strike drops the red cultist with a nasty slash across her throat, and the other manages to pierce the shadow and cause it to scream - an unearthly screech that echoes along the passage and instantly alerting the rest of the party that something is definitely wrong!
• Zyllyne feels a shadow flow past her into the tunnel back to where the rest of the party waits and decides it's near-invisibility just won't do. She starts to grow and causes a burst of glowing purple light to coat everything in a 20' cube! (Mulligan - make a DC11 Dex save. Red shadow(15) :: Blue shadow(11) :: Blue cultist(7))
• Dara hears the screech and rushes into the tunnel and sees Mulligan drop one cultist, and then attack a shadow. (Waiting on red shadow save to determine if Dara can see that shadow...) (Blue cultist DC15 Wis(13) His spiritual weapon then follows up the assault on the blue cultist, dropping the half-elf in the gray robe to the ground!
• Celeste sees the red shadow in the tunnel, blasting it with a luminous arrow. The radiant damage spreads throughout the shadow's form, pulsing and growing until the creature is completely dissipated! (Red shadow destroyed!)
• Vesta flings a firebolt into the ritual area, but can't see anything and it explodes harmlessly against the cave wall.
• The Leader steps out from cover, speaking a word of power directed at the expanding Zyllyne - "Flee!" (DC11 Wis save or spend your turn moving away from him by the fastest available means.) A moment later, a floating, spectral short sword with a wavy blade appears in the air between Mulligan and Dara, attacking the cleric! (Attack: 18 Force damage: 4) He then ducks back behind the rock and out of view from your current positions!

Map

Round 2:

• The blue shadow attacks Mulligan again, trying to absorb his life force! (Attack: 9 Necrotic damage: 10. On a hit, your Strength ability score is reduced by another 2 points.)
• Party up! 

Zyl save: 9

"Wha!? Ugh! I hate this place so much! Outta my way!" Zyl begins marching away from the cult leader, stomping back off toward the previous room.

Dara directs the weapon towards the looming shadow in front of him, its shimmering edge trying to strike the shadow in the darkness. Then he pushes into the room, trying to see where the cult leader is hiding. 

Toll the Dead on the leader: DC 15 Wisdom save or 1 necrotic damage

((If he can't see the cult leader from where he's standing, he will attack the blue shadow with his dagger. Attack: 20 (with disadvantage), Damage: 6))

Spiritual Weapon on the blue shadow: Attack: 10 (with disadvantage), Damage: 6 force damage 

     Mulligan attacks right back at the blue shadow, dodging out of it's way but then slicing right back right a psychic dagger...

Psychic Dagger vs Blue Shadow:  21 to hit for 7 psychic damage and 5 Sneak Attack damage.

     Hit or miss, Mulligan then scampers off up the wall and onto the ceiling using his movement to get away from the shadow and using his Bonus Action to hide amidst the shadows and cracks of the ceiling...

Nocking a pair of luminescent arrows in her bow Celeste creeps forward. "Denuh, denuh, denuh denuh denuh denuh denuuuuuuuuh denenenenuh..." She prowls like a pink panther stalking her prey. The druid takes a wide berth to see if she can line up a clear shot of the cultist, if she can't, she'll wait until they pop their head out.

Action - Star Map Guiding Bolt:
Attack: 9 if still DA: Nat 1 for a 6 Damage: 14

BA: Luminous Arrow
Attack: Nat 20 for 25 if still DA: 9 Damage: 6 if no DA and Crit: 6

Round 2 (con't):

• The blue shadow attacks Mulligan again, trying to absorb his life force! Missed!
• Zyl, for some reason, storms off back the way you came!
• Dara forces his way past the spiritual weapon of the cult leader and into the cavern. Dara spots the evil man peering around the rock and causes a bell to toll his demise (DC15Wis: 15). Dara's own spiritual weapon has no luck finding the hidden shadow attacking Mulligan!
• Mulligan swings and connects with his shadowy assailant, this time finishing the thing off! Mulligan then runs up the walls to hide. (roll Stealth, please!)
• Celeste follows Dara into the large cavern, heading towards the center before focusing her mystical aerial assault on the cult leader. The first arrow plinks harmlessly off the cavern wall, but it was only a ranging shot - followed by an expertly placed luminous arrow that hits the man square in the chest!
• Vesta ?
• Leader (-13)

Shaking off the confusion, Zyl will turn around still very angry and run back toward the leader.

Action: dash

Bonus action: Rage

(Mulligan Stealth:  22)

(So the blue shadow is down, correct?)

Vesta can no longer see the shadow attacking Mulligan, but she hears the sounds of combat coming from further down the corridor. She rushes past the spiritual weapon, which attacks her, but she keeps going until she is in the huge room. There's no time to look at her surroundings because there's still a cult leader to deal with. Vesta focuses her magic on him and unleashes an Ice Knife against him.

Attack: 22 Damage: 7 piercing damage and 7 cold damage on a failed Dex 15 save

Vesta runs into the cavern, getting to a point where she can see the leader around the jutting rock wall and unleashes a dagger of ice at him! (DC15 Dex: 5)

The cult leader surrenders, dropping to his knees, throwing his hands up, and dispelling his spiritual weapon. "I give up! You can arrest me..." he calls out piteously from the ground.

The runic circle on the floor suddenly hisses and flashes with lurid light. As if being extinguished, the glow of the runes dims as shadowy smoke rises from the carvings. The smoke gathers in the center of the chamber, where it coalesces into an apparition of an emaciated skull with one glowing green eye.

The apparition speaks in a hissing baritone. “Good news, Zalryr. Though your efforts have been disappointing, you have brought me new... points of interest.” The apparition swivels to look at each of you in turn. “Yes, there is great potential here. May we meet again—but until then, I have my eye on you.” The apparition then explodes into streaks of shadow, passing through each of you with a whispered scream before vanishing.

Everyone roll 1d20. If you rolled an even number:

 If you rolled an odd number:

Mulligan is odd - 19

18

"You! Why I outta..." Zyl stopped short from smashing the cult leader when the terrifying image appeared, made its announcement, and then vanished in a scream. Zyl was still screaming afterward for a moment before composing herself, slowly shrinking back down to her fun size. "So are we all finally ready to get out of here? Let's take this BAD MAN with us, maybe the Lord can get something more out of him. And...what was super creepy, right!? I don't like the idea of someone like that watching me all the time! What if I'm sleeping? Cooking? Taking a d....you know it doesn't matter. It's weird and I don't like it. Let's go!"