Cyanwrath moves the chest aside and pulls his greatsword from underneath, but not before Gunra rushes in and knocks him back to the ground. Kane's blast bounces off his scales, but the attacks of the rest of the group do not, and the Dragonborn looks heavily injured.

Still, he stands up, readying his sword. "You puny creatures think you can kill me? Me, the Champion of a Goddess?"

By the way he is talking, he seems to believe that Tiamat will keep him from being killed somehow.

Gunra's breath catches for a moment.  That sword had drunk deeply of his blood once.  Thankfully, he was not alone this time.  He again brings his axe to bear, counting on the half-dragon's overconfidence to allow another blow.

"But Champion, I survived our last fight.  Did she fail you then?  Or did you fail her?"

Attack: 7 Damage: 7

Now that we are engaged in melee, Petmailer draws his rapier and shield, and maneuvers to the right around Cyanwrath, moving into flanking position, facing him.  

He pierces him with his rapier :  Attack: 10 Damage: 6

As a bonus action he takes some deep breaths, gaining a second wind.  HP recovered :  6

"That's a nice sentiment you've got there," Harmony chuckles sarcastically as she whirls around for another strike.

Attack: 16 Damage: 9

Seeing as his companions start to "dance" around the dragonborn and engage him in melee, Nurm tries to distract the Cyanwrath's mind.

Mind Sliver: Save (Int): 13, Next saving throw penalty: 3, Damage: 1

Kane, determined not to miss again, fires another Blast at Cyanwrath. He sees that the Dragonborn is getting very weak and the group may be able to finishe him of here and now. 

Attack: 9

Damage: 11

 Posting...

Cyanwrath parries Gunra's attack with his greatsword, his smile showing a multitude of pointy teeth. "The mere fact that I exist means her will persists, I will not fail my Queen as long as I live"

He pushes Gunra to the side to evade the tieflings' attacks, and Petmailer's rapier just leaves behind a bleeding but still minor wound. He laughs mockingly, preparing for another short speech as rises his sword... When he suddenly stops on his tracks, a mild disorienting sensation blurring his sight for a second.

"I've had enough. Begone!" He roars, enraged. He inhales and breaths lightning at the party.

Lightning Breath: Langdedrosa breathes lightning in a 30-foot line that is 5 feet wide. Each creature in the line must make a DC 13 Dexterity saving throw, taking 21 lightning damage on a failed save, or half as much damage on a successful one

((EDIT: ...omg why))

Dex save (please dice gods I am the only healer do not let me go down): 14

(Is everyone hit by this?)

(Edit: Phew!! That would not have been good. Holding off on posting my turn for the moment to see who gets knocked out so she can heal)

Petmailer sees the bright blue light come from Cyanwrath Langdedrosa - 

Dex save :  12

And stares down at his chest as it goes right through him.  He gasps slightly as he drops to the ground, unmoving.

Dex save: 11 

Nurm sees, as the dragonborn opens his mouth. In the split moment Nurm realizes what's going on, it is already too late as the torrent of lightnng hits Nurm's body and the Gith falls to the ground

Petmailer death save :  19

Kane smells the lighting coming from Cyanwrath’s mouth. He tries to get out of the way...

DEX: 9

The lighting goes straight into him...he falls onto the floor, nearly dead. 

Death Throw: (please I don’t wanna die..I don’t wanna die..I don’t wanna die.) 5

Second Death saving roll: 3

Last Death Saving throw: 1

Death saving throw: 3

*Going back in time...*
Kane had been taking cover from behind a rock..and it was a good thing too. Kane began to smell the familiar smell of lighting and knows what’s about to happen. “Guys!! Get out of the way!!”

Gunra struggles to keep his footing as the powerful warrior unleashes his electric attack.

Dexterity saving throw: 10

Gunra feels the lightning coming on and digs deep for the aegis of Io.  He casts Absorb elements with his reaction.  He hears and sees his friends falling, but cannot help while Langdedrosa still stands, so he doubles down with his axe.

Attack: 18 Damage: 4 slashing and 2 lightning damage.

(I assume he's immune to lightning damage of course.