Jean grabs for Amaruve, but stumbles. Amaruve and Perspicacious are in the lead, Jean is just behind, dashing to keep pace. Gordra and Cordelia are perhaps 40 ft behind at the tunnel entrance.

Perspicacious- You bolt into awareness of your surroundings- you remember this place, or rather, your body does.You, Jean, and Amaruve are at the tunnel intersection. To your left and down about twenty feet is an iron grate. You hear a whisper in your mind as you look toward it, "Not yet". The tunnel ahead leads back toward the medica. The right turn will take you up and out into the town hugging the exterior of the arena; the setting sun casts a few deep orange rays down into the corridor... 

Jean and Amaruve- you are at the intersection, and that is all you know.

Perspicacious tries to direct Amaruve to the turn leading outside. "That way, follow the sun!"

Amaruve squeezes through the narrow halls, but still makes good time even in the tightest spaces, though he stays within sight of his allies to insure no one is left behind. (Using his action to Dash if necessary, or Dodge otherwise.)

Cordelia retaliates as attacks are targeting this dear old man. She draws her bow, targeting Ritz through the flames, even as she manages to keep her pace, hitting 18 for 8 piercing. "No time to stop and study your new toy now. Let's get out of here!" She calls to Gordra.

Amaruve:

Unseen by the fleeing party, Ritz is downed instantly by Cordelia's arrow (nice shot, BD). The Pretender swipes his hand at the gate of violet flames, and it is instantly extinguished. Glancing at his fallen attendant, he curses again, his voice raw after the alien incantation he just uttered. He softly mutters a command word to Ritz, even as his features begin to shift and change as the effects of his larger spell begin to take hold. He laughs, "I guess there's no point in hiding anymore. We'll take these fools, and then we'll raze the Duke's manor." He looks thoughtful for a moment- a true scientist, he takes note of the effects his as of yet untested final work are inflicting upon his body. Then he screams in pain.

Two horrible things happen at once.

The first is that Ritz's head pops off its knobbly shoulders with an audible SQUIP . Fine, threadlike tendrils begin to pull the nightmare fishbowl that is the homunculus' skull along the floor toward its master. 

The second horrible thing is that the Pretender explodes. The surface of his skin had been slowly roiling like water in a pot, but with a terrible thunderclap his body suddenly extends ten feet in both directions. Bright pink legs extend outward along his deep purple segmented body. At his front end, a vicious pair of black pincers extend 3 feet from a blind head.

Then Ritz reaches the carapace of the beast and is absorbed. All is still for a moment, and then black eyes open in unison all across the creatures body. It rears back, and with a terrible roar that shakes stones, it sets after the party into the tunnel.

(Updated Initiative order below:)
Gordra: 20
Dragon Centipede: 18
Perspicacious: 16
Cordelia: 12
Jean: 10
Amaruve: 10 

**Dragon Centipede?**

**Am I the farthest back at this point, since I stopped to do fire things while everyone ran past me?**

(yup, though Cordelia is by your side)

Gordra is running along beside Cordelia, trying to keep up with the warhorse.  At the sound of the roar he'll turn to see the abomination behind them. "Ms. Luna! Runnn!"

And he'll just Dash.

(That's my turn next, then)

The centipede is 100ft behind you after your dash. It also takes the Dash action, closing the distance to 50ft. It screeches at you, Gordra, pincers clacking menacingly.

(If you're looking for stats, this is a homebrewed monster. I've tried to keep it balanced for your level, but... you know, don't get eaten)

Ms. Luna does indeed run, but with her speed and cunning she is able to fight on the run. She Dashes firing another arrow behind her as she nears Amaruve, hitting 19 for 4 piercing. (Crit: 2)

...

Amaruve takes the turn ahead making for the smell of fresh air and the sounds of the arena. Maybe the pretender will like to take a more active roll...

(additional 6 against the dragon centipede (crit))

(I'm going to give you the 6 additional damage since I rolled it before seeing your 2- Cordelia for the WIN today)

Perspicacious will continue to hold on to the bear/horse, and will use the Help for Jean's next attack/ability check. Perhaps by steadying her as she shoots back at the... thing.

(Jean, because of Perspicacious' support, you now have Advantage (or can apply disadvantage to a save) in your next action)

(I won't reveal enemy HP, but just as a heads-up, a crit from Cordelia's arrow didn't seem to slow this thing down at all)

Jean drew her blade, sighted down the runes at the monster pursuing them and then used it to cast Command. "HALT!" She cried.

Jean's eyes and her sword flash with sudden power. Bringing to bear the weight of the Tower's authority, she speaks a single word and the monstrosity is stopped in its tracks just feet from Cordelia and the professor.

The way before you is clear. You have only moments before the effects of Jean's command dissipate and the monster is freed. The last light of the sun leads the way back to the world above.

Gordra just keeps running, I can't think of any clever tricks and I don't want to stop and attack. Run run run, to the light

Jean kept running to keep up with the others, hoping that soon they'd have some space to figure out some kind of plan.

(Sounds good. I think the main group paused just long enough for you two to catch up. We'll say the centipede is perhaps 120ft behind you now. The stairs are 50ft away. The centipede is just starting to struggle out of its bonds. While it is restrained, you all have one free round of action.)

Cordelia continues to run and fight, hitting 18 for 5 piercing. As she studies this fascinating magic... from a safe distance. (Arcana: 18)

...

Amaruve continues to carry his riders toward the exit. He considers what they're up against (Nature: 20)

Incredibly, Cordelia's arrow just barely makes its way between two plates of armor to bury itself into the flesh of the monster. Taking a breath as she continues to bolt for the door, she considers the transformation she witnessed as she and the professor stood behind the wall of violet flame. The Pretender was clearly a practitioner of transmutation magic, and the spell he used to turn into the dragon centipede (which she had just decided to call it) was not a simple use of polymorph: the man had invoked the energy stored in his lab to exceed the limits of his own power. Therefore, this form might be a) dependent upon the lab's ongoing functioning to be maintained, and b) not entirely stable outside of a certain radius, probably no more than a mile.

Amaruve, this is a creature of dark passages and an incredibly potent predator. As a rule, centipedes are venomous and creatures of the night. It likely will not want to venture forth above ground until after the last light of the sun is gone.