Nate is positioned perfect, although he'd be awkwardly trying to get behind Nub and at the last moment when it almost becomes obvious what he is trying to do (hide behind the tortle), he falls in line where he is currently on the map.

He whispers "did you see the size of it?"

Initiative: 5

Jalt is perfect where he is

13

Soar initiative:15

Nub initiative: 15

Soar is good where she is, as is Nub. 

I'm getting the red banner of death, so will re-roll initiative.

First roll: 8 
Second roll: 10 
Third roll: 22

Round One:
Blagothkus
Glazhael the White Dragon
Soar and Nub
Jalt and Nate
Wiglof and Hulda

Blagothkus moves up behind the rest of you, but cannot pass by you into the main chamber to face the dragon.  He frowns, and says:  I hope you have a plan to keep the dragon on the ground.  

Unable to reach or see the dragon, he takes no further action.

Glazhael sees the illusory party taking a menacing stance, and doesn't hesitate;  he opens his massive maw and a stream of cold energy blasts out, consuming the four non-existent adventurers.  He then takes off, massive wings driving blasts of cold air in all directions as he flaps up, now hovering thirty feet above the ground.  The adventurers of course are completed unharmed, and the dragon seems initially confused by this.

Soar and Nub are up.

(Also, can you please confirm all active spell effects:  Soar has Heroism, Nub has Fire Shield - who has Aid?

((Nate has protection from energy - cold))

(Soar does have heroism up. Nub doesn't have fire shield, since it's a 4th level spell. I believe aid is on everyone in the party but Soar?)

Aid on everyone but soar

(What level was Aid cast at?  If level one, then Nub, Nate and Jalt need to add 5 temp HP to your character sheets - if higher, then let me know.  I've added 5 to Nub for the time being.)

(it was at level 3 for 10 hp)

Added - thank you for the reminder

Nub uses his action to change the major image to have the adventuring party first momentarily grow icicles on themselves in reaction to the dragon breath, and then burst into flame with wings of fire and flying up above the dragon. They quickly become flaming scary demon-like creature adventurers. The dragon feels the temperature change. They quickly illusorily cast a web of fire spell that expands into a gently falling flaming web from above. They attempt to get it to land to avoid the spell and the flying scary flaming demon creatures who advance behind the "protection" of the spell.

He stays where he is.

-----

Soar looks up, and in her cultist voice, says "Intruders! They have made a fiery, unholy alliance! We will defend you, Glazhael!"

She steps forward, past Jalt, and attempts to climb some of the treasure pile, close to the dragon as she can get. Pointing the giant sword (she's left her shield behind, wielding the sword) at the illusion. She is fixated on the illusion, and tries to persuade the dragon to land. "Draw them closer so this blade may destroy them!" Persuasion: 27

She uses her bonus action to use hexblade's curse on Glazhael.

That's a hell of a roll...  We'll see how Glazhael responds on his own turn.

Nate and Jalt are up.

Nate will hold action and remains where he is, hoping to hell this works. He'd stay light on his feet behind Jalt in anticipation of the dragon's response. 

Jalt will also hold action.

Hulda and Wiglof look puzzled, but also follow your lead and do nothing.

Round Two:
Blagothkus
Glazhael the White Dragon
Soar and Nub
Jalt and Nate
Wiglof and Hulda

Blagothkus mutters something under his breath in giant, but he seems willing to follow your plan. 

Glazhael, surrounded by apparently flying adventurers bent on destroying him, lets out a growl of fury, and his head snaps in Soar's direction.  With a great blast of air from his beating wings, he lands atop the pile of treasure (it's about fifteen feet from the floor at its highest point and doesn't present a problem to move across or climb for you).  These are unworthy prey for me.  Slaughter them, and then we will discuss your punishment for failing to keep them from this place. 

6 

The dragon draws breath, and once again a blast of cold energy erupts from his mouth and consumes the four flying illusions.  When the air has cleared, Glazhael sees that they are completely unharmed...

Soar and Nub are up next.  Soar, you hear Hazirawn in your mind:  Strike now, while the beast is without its greatest weapon.  Strike!!

Nub moves past Nate and Jalt and hugs the wall as far as he can to the right/east, and blinks at Soar. He raises the staff and casts fireball from it for 29 fire damage, centering it just behind and above the dragon such that he keeps Soar out of the blast.

-----

Soar steps forward and says, "I'm not big on punishment, so I'll have to decline the offer. I have become more of a dabbler in beheadings, though, so let's try that..."

She darts forward and, if she can, tries to run up the side of the dragon to its back. (Realize this may not be possible.) Either way, she attacks swiftly with the new greatsword. She should have her hexblade's curse going, but if she wasn't close enough before to invoke it, she does it before she attacks. (+3 damage to target, and a crit on a 19 or 20)

Attack: 10 Damage: 13 plus another 6 necrotic damage
Extra Attack: 18 Damage: 15 plus another 7 necrotic damage

Level 1 smite on a hit: 7 radiant damage

(perhaps another on a second hit, editing...)

Wow. Both crit. So, doubling dice rolls for everything but what the roller doubled for the slashing damage on the first hit:
+2d6 for the extra necrotic damage on the first hit 7
+2d6 for the main sword slashing damage on the second hit 8
+2d6 for the necrotic damage on the second hit 3
+2d8 for the first level smite on the first hit 10
+3d8 for the smite I'm adding in now that I see the second strike was a hit 19 plus another 3d8 on that smite because of the crit 9

That feels absurd but check my reasoning here and let me know if I'm wrong...

Glazhael's DEX save for Fireball:  11.  He will use a legendary resistance to succeed, taking only 13 fire damage.

Soar runs close to the dragon, though not quite able to get fully around it.  Soar, you hear Hazirawn's gleeful laughter in your ears as you cut two deep gashes into the dragon's flesh and it roars in anger.  Hazirawn's necrotic power seems to flow through you, and you feel the evil, the pure, intense evil of the weapon you hold for the first time.  It's like nothing you've ever experienced before, not the undead, not the creatures of darkness you fought in service of Kelemvor - you sense Hazirawn consuming the dragon's life and savouring it.

The dragon will now use a legendary action to make a tail attack on Soar:

Attack: 31 Damage: 24

More hampered by its wounds than it thought, Glazhael's tail slams round but flies over Soar's head.

Jalt and Nate are up.

(Assuming Im correct with the squares = 5ft) Jalt will rush towards the White Dragon and casts Bane lvl 1  CHA 14 save and yells out to the dragon in draconic, WUX VUR DOUT DAARIV GEOU WIELG EKESS WER JANIK DI BAHAMUT!!

For those who speak Draconic