GB will cast [spell]Magic Missile[spell] (Lvl2) at the dragon.  Fearing other spells would hit Bel.

15

(Nice roll!)

The darts of force slam into the dragon, annoying her something fierce, and she locks eyes with the flying Aarakocra wizard...

(Lana is up!)

Lana will use a bonus action to cast Hunter's Mark and use her longbow to attack the dragon.

Attack: 12 Damage: 9 Hunter Mark Bonus 2

The magically enhanced arrow strikes the dragon's black scales and sticks in, but she is still focused on the person on her back. Taking a different tack, she flies to the west and dives underwater!

// Bel, you have the option of letting go (just a reaction, roll a d8 to determine where you land) or holding on (make Strength check). Let me know what you'd like to do! //

Seeing the dragon head for the water, Bel leaps from her back. 7 Where he lands.

Bel lands smack atop a pile of coins next to Aregarak’s unconscious body. As he lands, the silver and gold shifts under him, and he upsets a potion bottle, which rolls down the small pile of coins. As it rolls, its magical red liquid begins to glimmer in the dim light.

The dragon plunges underwater, all but disappearing into the black lake.

// Players up! //

[Death save: 20]

"Uhhhhhhhhnnnnnngggggghhhhhh........ouch....." as Aregarak sits up.

// Aregarak can take his turn as normal! (Starting prone of course) //

He slowly staggers up to his feet, still a bit unsteady after nearly dying, as he raises his head, he spots the potion bottle that was rolling around.  He grabs at it, and decides to roll the dice because he's in no shape to continue fighting a dragon, pops the top and downs it in one gulp.  Then hefting his greatsword up to a readied position he prepares for the dragons next onslaught.

"If I am to dine with Talos this evening, it will be on my own terms!" he shouts to no one in particular  "not because some overgrown lizard wills it!"

Aregarak experiences a sudden surge of energy as his wounds begin to mend of their own volition. 

Healing: 10

// Bel, GB, and Lana are up! //

“Nice to see you back my friend” as Bel stands and looks for a place to make a stealth attack from.

Stealth 15

Bel sees to the north and south, a short distance across the water, series of tunnels to hide in. Besides possibly going underwater, these tunnels are the only areas that would give any cover. 

//Which tunnels would be closer to Lana and GB?//

The southern tunnel is closer to Lana, and would take two turns to reach. The northern tunnel is closer to GB, and would take one turn to reach. 

//well, I guess it depend on the Big Guy. If he stands and fights then Bel will stay at his side otherwise he runs for the southern tunnel.//

// Aregarak, feel free to let Bel know what you’re doing! 

GB and Lana can take their turns as well. //

With his pouches full of whatever he could find, Aregarak looks around one last time for anything 'expensive' (like maybe that spear if nothing else) grabs it and heads back towards the caves they came out of.

"Run while you can! the witch lied to us!"

Hearing the Big Guy yell, Bel makes a run for it screaming to the others to follow.

GB will head to the tunnel with the rest of the party but will ready [Tooltip Not Found]Lvl 2, if the dragon come out of the water again.

If he comes out of the water again he will take 13 dmg from spell.

Quote from Jcburgg >>

GB will head to the tunnel with the rest of the party but will ready [Tooltip Not Found]Lvl 2, if the dragon come out of the water again.

 // What was that spell? The tooltip didn't work :) //

EDIT: Never mind, magic missile, got it.

Quote from ArgentTheWise >>

With his pouches full of whatever he could find, Aregarak looks around one last time for anything 'expensive' (like maybe that spear if nothing else) grabs it and heads back towards the caves they came out of.

 The spear shimmers in the torchlight; it is made of exotic materials and is obviously magical.