Shadowthorn's Princes of the Apocalypse

You've been fighting on the causeway—the dragon just happened to be perched there, but they moved forward before the fight even started.

Wait, if it's a causeway? Then it's it's not too far of a drop, right? (And we're already underground.) Kethra will be taking the dragon for a drop off the edge so we both fall and end up prone. Could we get a map of the causeway? Then Kethra stands up. 

By causeway, I mean it's essentially a bridge over the moat and it's like a 5/10 foot drop into water.

Here's an image of the area, the little circles are ornamental pillars and the blue is a tiled plaza in the distance.  The pyramid is behind you (I actually confused myself, I thought it was in front of you), so to the left out of the picture..

With the causeway after the pyramid, then can Kethra go ahead and throw herself off the pyramid with the dragon in tow? We'll both be taking the same damage anyways.

(Again, you're not on the pyramid.  You're on the causeway.  The dragon was perched on the pyramid, and flew down.)

(The wyvern is up, just for sure want to know where Kethra is moving.)

I'm sorry for resorting to tactics that are reliant on terrain while playing theater of the mind by text, but fighting a creature with +7 to hit (as in, even if you had 17 AC it just needs to roll a 10 or higher to hit you) or take 60 damage means Kethra should try to buy as many turns as possible for everyone else to attack.

Quote from LordShadowthorn >>

for sure want to know where Kethra is moving

I'd like to tell you, if you were more clear on exactly *where* is! -_-

Now if not the pyramid, then the causeway: is it 5 or 10 feet of a drop? 

Quote from LordShadowthorn >>

By causeway, I mean it's essentially a bridge over the moat and it's like a 5/10 foot drop into water

The whole plan, which I've already explained above, literally hinges in this detail...

(Let's put it this way—the dragon is not going to take damage from you dragging it into the moat, whether it's 5 ft or 10 ft.  The moat is 10 ft below.)

(If by this you mean the dragon's gonna try to fly, being under the grappled condition means the dragon has 0 feet of movement so it can't fly to catch itself. It can roll to take half damage tho, if you want to use Tasha's optional rules for falling into liquids/water. Link.)

(Here's the rest of my turn. If you still don't like it LST, just let me know and I'll change it. I'd love to work this out together with you, but I also would, if possible, try not to die from this 😔)

With her free hand, Kethra points to Marros and Xanranzan, then points to the water. "Marros, Xanranzan! If nothing else comes out of this water, you two follow me and use your swords...I'm sorry to ask this of you, Xan..."

Next she points to Mr. Tyde, "...Mr. Tyde, you're doing great, keep casting spells!"

Then she nods to Rubin, "If you've got a knife, come follow us in. This is now or never!"

Lastly she smiles at the dragon. "Alright big boy, let's see how those wings work in the water!" Kethra drags the large creature off the causeway and into the moat (10 feet doubled to 20 feet of movement while dragging a grappled creature)

• 10 ft of falling damage: 5, condition prone in water.

She'll use the last of her movement to end the prone condition on herself and then strike the still prone dragon with a bonus action.

• To-hit: 18 Damage: 9

The wyvern struggles in protest, but is dragged down in to the water (13).  It struggles against Kethra's grapple, and blindly lashes out at her in a rage.

Bite: 19 Damage: 15

Stinger: 20 Damage: 13 and con save for half of 22 poison

(Everyone is up)

One of the last thing Kethra sees before jumping into the water is this expression Xanranzan's Face:

Yet when push comes to shove, Xan calls out an "I-I-I, ah, sssss-fudge monkeys on a gold vien! Hold on, Kethra! I'm coming in!" And with that, he dived in and let natural density help in sinking down fair bit before working his legs in over time to get in close and start try carving up the wyvern. Unfortunately in the water, his blades weren't of too much help. But with the beast floundering even worse than he was, at the very least the playing field was somewhat even!

Action: Attack w/ Saw-Toothed Blade.
To Hit: 13 For: 18 magical slashing damage.
(If a hit, spending Defensive Flourish to add 5 to the damage and add the same amount to Xan's AC)

Bonus Action:  Attack w/ Off-Hand Longsword.
To Hit: 16 For: 11 non-magical slashing damage.

Rubin frowns at the water and then at his crossbow. There is simply no way he is following them into the water. He holds his crossbow ready to shoot if the dragon-beast comes out, but otherwise stays put.

Quote from LordShadowthorn >>

The wyvern struggles in protest, but is dragged down in to the water (14). 

(btw just for next time, the dragon's check should be at disadvantage from Marros' Hex spell)

Marros idles up to the edge of the causeway and peers over the edge. A thrashing dragonlike creature desperately trying to escape the grasp of a saturated Kethra is all the greets him. His nerves suddenly fire. "You two look like you've got it pinned. I'll stay up here with Rubin!" With that he unleashes the now familiar bolt of orange light at the dragon.

Attack: 23 Damage: 9 plus 6 Hex damage

While Xanranzan moves in and strikes, Kethra deflects the proned creature's claw to harmlessly splash into the water (see picture, but with one hand) and attempts to counter with her own sword:

• Saw toothed sword to-hit: 12 Damage: 15 (Extra 1d8 from attacking an unarmored creature, and extra +2 for rage)

"Stormwind Secret technique #16! Dance of Life and..."

The creature bucked Kethra into the air like a horse but she managed hold on and swat away the tail attack with a horizontal parry. She tries to knee to the side, then bring her sword arm's elbow down on the creature's head.

• Flurry of Blows 1: 18 Damage: 7
• Flurry of Blows 2: 8 Damage: 9

"...death!" She ends, landing in the water adjacent to Xanranzan.

Edit: add a +5 to hit for the sword attack because I'm bad at math and an extra 2 damage for the second flurry of blows for the critical hit

Even through the massive beating it's just taken, the wyvern is somehow still just barely hanging on.  It lashes out desperately once more at Kethra, with an air of finality.

Claw: 13 Damage: 8

Stinger: 19 Damage: 10 Con save for half of: 27 poison

 Unlike on the surface with his feet planted firmly on the ground, there was little in the way of elegance to Xanranzan's movements while trying to cut down the draconic creature. He lashed out instead more like a madman desperate to be away from the immediate circumstances, but knowing or refusing against better judgement to flee from the surface. Had the wyvern been any smaller, his hacking away might've very well endangered Kethra in the process. 

(The below assumes Kethra has no way to up her AC or reduce the poison damage. Because even taking half from the piercing, that would be 21 damage minimum. D:)

It's only after expended himself in flurry that a new kind of fear sinks into mind seeing Kethra began to drift away unconscious. "Keth-thrubb!" He shouts before thoughtlessly diving down trying to grab her, even if that meant abandoning his sword or taking a swipe from the Wyvern.

Action: Attack w/ Saw-Toothed Blade.
To Hit: 18 For: 8 magical slashing damage.

Bonus Action:  Attack w/ Off-Hand Longsword.
To Hit: 18 For: 11 non-magical slashing damage.

rubin sends bolts in if he can get a clear shot at the wyvern from his place ont he solid ground above.

Attack: 14 Damage: 4
Attack: 14 Damage: 5
Bonus action Attack: 9 Damage: 5

Quote from Brian_Avery >>

"Hold it still! We'll all strike it together and finish it off!" Tyde yells for everyone to kill it quick.

While still hanging on to the dragon, Kethra kicks away a claw attack and lets out a nervous laugh.

"Hey, I think we might have this one in the ba--," she suddenly falls silent in mid sentence, and her body falls limp like a doll. The still-dripping stinger of the dragon remained in place where Kethra once was.

Quote from Mehmen >>

(The below assumes Kethra has no way to up her AC or reduce the poison damage. Because even taking half from the piercing, that would be 21 damage minimum. D:)

(Uhh yeap looks like you're right I think the mathematics checks out. Even if Kethra passed the save she'd still be 0 HP. At least its not enough damage to instantly kill her. ´･_･`)

 Death save: 17

The wyvern downs Kethra in its vicious last stand, but is immediately finally taken down by Xan and Rubin.  Kethra, however, is not doing well.

(In case you were wondering, it lived with like 4 hit points last turn.)

The dragon creature finally succumbs to the group's efforts, but at a huge cost. Kethra. Marros watches as her body sinks in the water - the following moments all seem like a blur to the Warrior of Light.

One moment he's standing atop the causeway. The next he's jumping over the edge and splashing in the water. He reaches deep into the water and draws the unconcious head of his team mate to the surface. "No lady warrior. You do not get to leave the Light so soon!" A golden light forms in his palm as it supports her unmoving head. The glow spreads down her neck and across her body.

((Bonus action: Healing Light (2d6) for our fearless dragon grappler))