(Boona may also act if she's up... Or roll a death save if she's not...)

( I'm 95% sure she should still be up so I'm going to roll with that. Also not sure if Boona has disadvantage as she knows where the snake is so I'm gonna go with straight roll.)

Boona was slammed by the big snake and felt it wrap around her trying to drag her down and decided two can play at this game.

She seized the snake in turn and shoved it away from her

Grapple- Attack: 19 Damage: 6

If successful the snake has a DC 15 check to resist the shove and Boona moves 35' towards Bryony.

( Not sure why the error message appeared just edited the last line, roll was originally a 15.)

Finally pulling himself out from the ravine, Ruven checks himself over, muttering curses in several languages. This was supposed to be a routine research expedition...and now he's almost died multiple times. He was beginning to take this personally.

Gathering himself, he hears shouts and cries of alarm and begins moving towards the group at the water.

Init 17

The snake, grappling Boona underwater, is hit by both Bryony's carefully-aimed shot and Jurgen's wild sword swing. It begins to bleed a thin stream of black blood under the clear, dark water.

The snake is too big to be affected by Boona's shove, alas. (It is Huge, two sizes larger than Boona. Shove vs. Grapple is an interesting DM conundrum, which I can fortunately sidestep for the moment...)

Dusting himself off, Ruven begins to race toward the distant sounds of commotion, catching Quinn's eye in the hallway.

The snake pulls Boona even deeper – nearly 20 feet down. Her torch extinguished, Boona is blinded as well as grappled and restrained.

Then it bites her...
Bite Attack vs. Boona: 18. Hit: 10 piercing damage.

Everyone may act!  (Underwater combat rules apply.) (Ruven is 35 feet from seeing even the bubbles, and Quinn is 65 feet from visual range of the pool.)

Quinn see's Ruven running down the hallway to where he last saw the rest of the group and he darts after him.

OOC: Quinn uses dash as close to the action as possible.

Boona- Death Save 1- 7

( Underwater....not sure if death saves are rolled or just auto-failed cos she can't breath...)

Jurgen and Draco continue their attacks.

Jurgen misses but Draco hits (22 to hit with 8 damage)

Sensing trouble, Ruven's rushing suddenly becomes more fluid and a soft glow envelopes him as he enters into Bladesong (movement now 40ft).

He reaches the stairs, assessing the situation. 

(Can Ruven see the snake enough to magic missile it?)

"I'm not losing another friend!" Bryony screams. She runs and cannonballs into the water, unsheathing her blades at the same time.

The halfling swims up to the snake and starts stabbing in an effort to get it to let go of Boona! (Her weapons deal Piercing damage so no disadvantage, yay!)

Shortsword - 9 to hit, assuming a miss? :(

Dagger - 10 to hit (wow....), 8 damage if it hits

Sneak Attack - 6 damage if the above attack hits

Quinn sprints down the hall after Ruven, leaping down the stairwells.

Ruven makes it to the final staircase but sees nothing but bubbles and waves from a violent disturbance somewhere within the dark pool (he may still have his action.)

Bryony dives deep but is unable to connect with the dark, squirming figure. (She is also blinded under the water.) Nor is Jurgen successful.
But Jurgen's steel defender, sinking like a stone to the bottom, manages to tear into the massive snake holding Boona's limp form.

Ruven may still act if he chooses!

The snake snaps at Draco before dragging Boona deeper to the northwest.

Bite Attack vs. Draco: 26. Hit: 10 piercing damage.

Jurgen and Draco, and Bryony (the latter blinded, with disadvantage) may get an opportunity attack as it retreats.

Boona wakes up, and finds her lungs filling with water, her body constrained, unable to see. She has only a few (well, 18) desperate seconds left.

Everyone may act.

Boona is still trapped and gravely injured ( 1 hp) but drowning must be her first concern....she wracks her brain trying to think and in desperation asks the water itself to help her.....

( Action- Attempting to use Elementalism to shape the water around her face and in her mouth into a hollow sphere around her head. Lasts one hour.

)

Draco, with that last hit, falls quite. 

Jurgen strikes out as a reaction to the retreating creature. 17 to hit with 10 damage.

Bryony lashes out with her shortsword as the snake swims away, but the water is dark, and it's hard to see. She's not sure if she actually hit the snake or not.

Shortsword - 12 to hit, 9 damage

Bryony swims toward the snake as fast as she can, hoping to stab it a few more times (with disadvantage).

Shortsword - Nat 1 :(

Dagger - 14 to hit, 8 damage

Sneak Attack - 6 damage

Quinn rushes around and runs to the waters edge. He looks around but see's no one. He yells.

"Bryony!...Jurgen!...Boona!"

Quinn looks down into the dark pool and thursts Lightbringer into the water to illuminate the area.

OOC:

DM If you don't require a perception check or any action to look into the water, Can I reserve my bonus action to dash and dive into the water if Quinn see's anything in the water?

Draco sinks to the bottom, where Jurgen spots an old skeleton lying on the bottom. Several arrows remain lodged in the skeleton’s ribcage. At a very quick glance he can see that it wears several jeweled rings and clutches a wand in its bony fingers. Jurgen spins and manages to stab the fleeing serpent.

Bryony does, too, chasing it blindly and nearly severing it. It is quite badly wounded.

Quinn sees nothing, and chooses to dive in, swimming toward the commotion.

Jurgen used his reaction, but may move and act. Ruven, maybe 40 feet from the invisible underwater melee, may also act.

Jurgen will make note of where Draco dropped. 

Jurgen will continue to chase after Boona. If near the snake he will attack. 13 to hit

(13 hits! It's big and unarmored. Roll damage.)

14 damage

And with that, the massive snake is slain. It immediately loosens its grip on Boona.

Everyone may act.