Mirica, untouched by the magical blast of cold, gets near to Toc, tapping his shoulder, protective energy flowing into him.  Then she holds forth her hammer and sunlight fills the space that is already glaringly bright due to the flashing colorful wall. 

Jahren's wound continues to bleed 4 damage. He calls out to Chen and Sebastion, who can hear him through the crackling energy of the wall, then, pulling out an instrument, but blind, he attempts to target a powerful spell, strumming a few strings that release powerful musical energy.  The whole area around the enemies fills with more streaming, colorful light, the musical energy adding to the hypnotic swirls, attempting to ensorcel the enemies. 

Aldlas : 11 LS2

Knight : 7

D2 :  11

D3:  8

D4:  7

 Aldlas hisses as the combination of magical light and music hits the enemies like a wall.  The undead wizard shakes his head, about to fall under the charm, but then somehow manages to resist it. The Knight stops moving, as do two of the demons.  Only one demon remains moving, hissing at the magic around it. 

 The demon opens its maw, and the whole space fills with a poisonous cloud.  Everyone at the start of your turn, make a DC 11 CON save.  28 poison damage, save for half.  

The poison hits the Knight who had just been struck by the spell, and the poison seems to shake him from his reverie (Con save: 20).  He charges forward, and ignoring Toc goes straight at Mirica, his Glaive tall and stabbing from a distance.  As he approaches he seems to summon glowing energy around his glaive, (casts Blinding Smite), and the tries to strike you.

Mirica: Attack: 21 Damage: 11. Attack: 16 Damage: 14. Attack: 9 Damage: 7

(if any hit, you take an additional 17 radiant damage, and make a CON DC 13 Save or be blinded)

Fianna, Chen, Sebastion, your turns now. 

Sebastion : you remain blind. take 3 damage from your demon wound.

Chen: as you stand in front of the wall, the flashing colorful lights dazzle your eyes and you're left staring at blotchy blackness.  You are blinded. 

Fianna : make your cloudkill save or take damage.  I'll resolve your action immediately below. 

Mirica:  take an addition 20 radiant damage from that first hit, since it was a critical...zoinks.

  [ Round 2/3: Chen: 21, Sebastion: 18, Adlas : 18, Toc: 16, Jahren: 12,  Mirica : 12, Demons : 11, Knight: 10, Fianna: 5] 

"Don't hit them if they aren't moving!" adds Jahren after some success with his spell.

Rolling vs the poison now before I forget: 10

Still blinded Sebastian lays hands upon himself for 1 point healing the wound

The Mastiff growls and is about to leap at the undead Aldlas and rip into his leg, ultimately dragging him to the ground like a common thief....but...then the poison washes across the room. The dog lets out a single yelp and drops to the ground in front of Aldlas with a heavy thud.  The undead looks down and grins. 

Then Fianna summons her Fey powers, the powerful dark energy rising in her being as she raises up her middle finger at Aldlas.  He grins at her.  The dark energy swirls and leaps towards him, but rather than assaulting his being, it seems to funnel in the air before him and like a vortex it swirls into the rod in his hand, leaving him untouched.  

Chen and Sebastion can hear Jahren and the others in battle on the far side.  The air full of arcane words as magic is flung in the small space beyond the flickering colorful sphere. 

(Let me know if you do anything else you two.....)

Aldlas raises the rod in his hands, and casts a common spell Magic Missile.  Seven (7) darts fly from the staff.  Two strike the stunned demons.  Zapped, they shake out of their reverie.  The other five go flying and strike Mirica as both the Knight and Aldlas seem to direct their attacks at her.   Fianna, as Mirica collapses under two massive strikes from the Knight.

Fianna : 12 force damage from Magic Missiles. 

D2 : 4. D4 : 2 

Toc, Jahren, Mirica's turn.  (remember your poison saves)

  [ Round 2/3: Chen: 21, Sebastion: 18, Adlas : 18, Toc: 16, Jahren: 12,  Mirica : 12, Demons : 11, Knight: 10, Fianna: 5]  

(But I do not have that many HP!)

Poison save: 15 , Save vs blind: 23

(Actually, Critical is already counted in the damage - double roll plus modifier, is not it?)

Toc glances back as Mirica falls under the Knight's blows..... a helpless look on his face as he faces down the two undead......"Jahren...please....help her....."

He leaps at Aldlas, re-doubling his attempts to take the undead wizard down with his blades: 

Attack: 21 Damage: 15. Attack: 16 Damage: 14. Attack: 19 Damage: 9

Poison save: 23

His blades rip into the undead wizard, over and over, sending chunks of undead flesh flying.  The fierce attack leaves Aldlas looking a bit shaken...and quite badly injured. 

Jahren? 

"You got this Fianna.  I've still got a few tricks for this ugly bastard!"  Bardic Inspiration: 1d10

Now choking on the poison, Jahren rushes to Mirica and both are gone in a flash (Dimension Door), appearing behind Sebastian along the back wall of the library.  An exhausted Jahren winces as he looks at the bleeding wound and looks to Sebastian and Chen.  He then speaks with more command and authority than either of them has heard from him previously.

"Seb, I need healing, and then so will she!"

Looking over to Chen he can tell something is wrong and thinks of the light and the potential blindness.  "Chen, back here to me, quickly!"

"Where are you??"

"Here, here, behind you" Jahren calls, trying to ensure Sebastian can hear his voice.  "Come to the back wall!"

Sebastion sees Jahren and Mirica appear on the floor in the library.  Mirica has two gruesome slashes across her midsection and looks near death, while Jahren looks pale and a bit shaky....though his commanding voice and presence belies none of that as he issues orders.  Chen, stumbling back, hears Jahren call for him. 

Then, out of the dark doorway to the north end of the library stalks four relentless shock troop demons..... but it takes Jahren and Sebastion only a panicked heart beat to realize that they all look grievously injured, wounded in the exact same places, and are with all the same scars and marks.....and so this must be a mirror image spell at work.  Regardless, the demon charges out at the group, though to Jahren's relief the demon charges Chen, seemingly enraged by the warriors on again / off again battle the two have been waging. 

Jahren sense that the demon, despite being on death's door, seems to be wanting to fight this battle to it's gruesome end at this point.  

A claw lashes out at Chen : Attack: 25 Damage: 12 (if it hits, make a CON DC 16 save or it will continue to bleed), followed up with his axe swinging around hard.  Attack: 22 Damage: 27

Sebastion, Chen, you may take your actions. 

Meanwhile....in the other room....

The knight turns his attention to Toc, his glaive glowing once more as magical energy imbues the weapon against the elven bladesinger: 

Attack: 20 Damage: 6 

Attack: 8 Damage: 9

Attack: 19 Damage: 14

+ 10 radiant damage on the first hit + CON Save DC 13 or blinded). 

The glowing glaive slashes downwards, but Toc, a blurry dancing form, seems all but impossible for the Knight to hit.

Meanwhile, three demons advance......two swing around and get behind Toc to attack Fianna: 

The first attacks with a claw : Attack: 26 Damage: 11 then another Attack: 15 Damage: 8. 

The next one attacks with it's two claws :  Attack: 26 Damage: 10.  Attack: 10 Damage: Unable to parse dice roll. 

The third demon moves towards Toc, and takes a couple wild claws at him : 

Attack: 12 Damage: 9 Attack: 17 Damage: 6

...failing to hit the blurry elf. 

Fianna, Chen, Sebastion are all up. 

Round 3/4 [ Chen: 21, Sebastion: 18, Adlas : 18, Toc: 16, Jahren: 12,  Mirica : 12, Demons : 11, Knight: 10, Fianna: 5]  

     Fianna Con Save: 16  

     My actions coming shortly! 

((Poison and MM damage taken))

     Fianna setps up and puts a hand on Toc.  "Toise doras dúinn ar shiúl ..." Fianna whispers and the two are immediately standing between Chen and Sebastian. (( Dimension Door obviously)). Put us standing against the far wall from the sphere...

((If Fianna can do so with a Bonus Action, she will drink one of her Potions of Healing...))

Blindness save 12

Cure wounds level 3 on Mirica. 20

Mirica's eyes flutter open as Sebastion ignores the demon and leaps across the room, out of the range of the blinding light and casts his spell on the priest.   Mirica finds herself on the ground, between Jahren and Sebastion, in the library.  The glaring rainbow wall fills the doorway into the Crypt room...and a badly injured, but still fighting huge demon is here battling a Chen who seems to be unable to see. 

<insert Chen's action here>

Fianna, with two bug creatures clawing at her and Toc also under assault, and their allies gone, summons her magic, grabbing Toc and...... Aldlas mutters a single word that echoes through the room and Fianna finds her spell cancelled......   (Counterspell at 4th level, his Arcana is high enough no roll is required to identify a 4th level spell and up his counterspell to respond.   I believe drinking potions is a standard action, but we can discuss on the chat if you like.)

Then, Aldlas backs out carefully away from Toc, using the Knight for cover, and backs away completely, vanishing through the rainbow wall of color.  Toc, unable to pursue, curses in Elven, glancing around at the plenty of foes around them....and still the poison choking both Fianna and him. 

He turns to one of the demons, glaring at it, and attacks. 

Attack: 32 Damage: 12. Attack: 28 Damage: 14. Attack: 27 Damage: 7

(Concentration on Cloudkill : mm damage : 12 DC 10,   sword hit: 7 DC 10, next hit 15 DC 10)

Aldlas poison save : 20

Toc poison save : 22

The poisonous cloud around Fianna and Toc vanishes.  (re-rolling poison dmg : 27  )

(Face palm, I ruined everything by trying to fix it.  Toc hit's the demon twice, and then it fails a con check, takes 19 damage, and the cloudkill goes away.  Toc and Aldlas both save and take a bit of poison damage.)  

Waiting for Chen's action, then it's Jahren and Mirica. 

Knight Con save : 20. (forgot his poison damage last round, amended)

Chen's plan of attack is to ATTACK!

RAGE! (Final Rage)

Action: Reckless Attack (to cancel out the disadvantage from blindness) against the nemesis demon
Attack#1: 28 Damage: 19 slashing
Attack#2: 26 Damage: 17 slashing

If there's a CON save for the blindness... 29 ... and if that's enough, and attacks have advantage instead of not... 22, 21  (Mostly just hoping for a crit)

Also, CON save from demon's last attack: 11. Is it one save for each hit? If so: 13

Wow, these dice are pretty hot.

Mirica gasped , coughed but tried to stand with Sebastion's help. She breathed again such a sweet not poisonous air (Second Wind for 10 ) but then realized, she is on the other side of the sphere now, with others still dying in a helpless battle.

Before doing anything else, she turned around, instinctively finding east, dropped to her knees and whispered a short prayer to the God, one you do not repeat often: "Hear me, Lathander, Great Lord of Light! In my hour of need, answer my call to you. Shine your light down on me, give me the strength to destroy the darkness that threatens us! Help me to protect those fighting against it. Let your light to be my sword and my shield in this endeavor, Lord!"

(Divine Intervention. If I read it right, I suppose to roll 12 or less on d100 to get it. As hopeless as it is - here it goes 2 )

Chen (who is not blind, sorry, i mistook a Con save for a blind save) turns to the demon and with two great sweeps of his axe lops off a leg, and then then creatures head.  The decapitated beast thumps to the ground in a pile of gore.  (only save for the claw attack, which you made, no need against the axe attack). 

Then he turns as Mirica, facing east on her knees, says a quick prayer.  As she prays to her god the entire space, both in the library, and in the crypt room begins to glow, the air sparkling as light begins fill the space. Soon the light is so bright, clear and white that it's almost blinding. Reality seems to fade away until there is only light, only a great white expanse, with nothing else to see, not even your own bodies....everything is just white.  And then, as quickly as the white light came, it begins to fade, the world around the group begins to reform, reality reasserting itself. Finally the light fades away and the world is once again as it was. With one key difference. The prismatic wall is gone. Standing just inside the crypt room, now revealed to all is the undead wizard Aldlas, who glances around at where his rainbow colored sphere once stood...."what.....impossible.....there is nothing that could achieve that!!!" he practically shrieks, rioting against the air around him.  Mirica, you feel your wounded body heal in the bright light. (You gain 21 HPs.) 

Fianna and Toc can see their friends, across the crypt room and through the doors....though they are still in combat with a number of demons and the knight.  

(so, both the Prismatic Wall AND the Cloudkill spell are now both gone)

Jahren, your action.  Then the demons and the Knight get to go. 

Round 3/4 [ Chen: 21, Sebastion: 18, Adlas : 18, Toc: 16, Jahren: 12,  Mirica : 12, Demons : 11, Knight: 10, Fianna: 5]  

Jahren smiles widely and says sincerely to Mirica "now THAT was something I have very seldom seen in all my travels, my friend." 

Seeing that the path is now clear to the archmage the smiling bard adds "hey Chen... give 'em hell!"  Bardic Inspiration: 1d10

The weary bard then takes a moment to himself, a moment of relief, and drinks a (greater) potion of healing before rejoining the fight.

Potion: 11

The demons glance around, the bright multicolored hues of light suddenly gone...seeing the left leg of their leader on the floor near Chen in the other room.  But they glance towards the Archmage and then their emotionless black eyes turn back to the battle. The knight hisses an undead breath at Toc, glancing around at the fight that has suddenly become far too fair for his liking. In some ancient tongue he mutters words, backing away from Toc easily, since his Glaive has kept him afar, and turns, marching across the room to stand between Aldlas as the rest of the group.  As he does so, he seems to focus and a wave of menace ripples from him. 

Chen, Fianna, Toc.  DC 13 Wisdom save or become frightened for 1 minute. (Dreadful Aspect Ability, not a spell for those with advantage saves against spells.) 

Toc : 14

One of the demons glares at Toc and Fianna, and the space around them fills with darkness. Fianna sees through the darkness, but Toc seems to have an issue with it.  Then one of the demon sets upon Toc, and the other sets upon Fianna. 

Attacks on Toc:  Attack: 10 Damage: 12 Attack: 21 Damage: 10

Attacks on Fianna: Attack: 9 Damage: 7 Attack: 9 Damage: 9

Fianna, Sebastion, Chen, your actions. 

Round 4/5 [ Chen: 21, Sebastion: 18, Adlas : 18, Toc: 16, Jahren: 12,  Mirica : 12, Demons : 11, Knight: 10, Fianna: 5]