Jan will drop down and move to the right and back, and taking Marrin's call to Ava as a suggestion, casts a spell, making a fist and barking out a command as she extends her fingers, the ghasts getting battered by thunderous energies that burst forth.

CON Save vs 14 or 17 thunder damage, half on success.

((Jan's last spell slot, but figure Marrin has more flexibility on what to use her remaining slots for if only one is needed?)) 

The piercing, ear shattering noise, centered around the base of the forward mast affects all four of the ghasts, though two seem to have been slowed even more than the other two.

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Avaria, Marrin, and Ethel are up.

Taking a deep breath, Ethel drops down as well, though she lands fall less gracefully than does Jan, her legs collapsing and leaving sprawling in the dirty water.  With only enough movement to rise to her feet, she weakly lifts he walking stick and casts web on the bow of the boat.

[I think that will hit all of the ghasts, without Avaria.  DC 15 DEX save or they are restrained.  Does difficult terrain stack?  If so, any who resist could face double-difficult terrain.]

Two of the ghasts get stuck in the webbing as it stretches around the mast and ceiling of the hold (Ghasts 2 and 3). The other two are nimble enough to dodge out of the way as the sticky webbing appears.

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Avaria and Marrin are up.

Will rule that terrain is either difficult or not. At some point you could probably do enough to make an area impassable but the water and webbing aren't enough for that and the area will just still be difficult.

Marrin hops down and moves opposite Jan but whistles lightly, sending another blast of sound on top of the ghast/ghouls.

Shatter (DC 15 con, half on save): 13

Ava stares at the four ghasts that emerge from the water and curses under her breath.  The thought of the stench surrounding her is all she can think about and wonders if she should climb back out of the hold.  

But then three splashes behind her surprise her.  She briefly turns to look back and sees that her friends have come to join her and it does so much for her resolve.  Turning back with an almost maniacal grin, she starts changing.  Shifting into her more beastial form (4 temp hp and no adv against her even if they flank) she holds her ground where she is, making sure she is between the creatures and her friends.  Listening to their warnings and wanting to steer clear of the smell, she pulls out a javelin and launches it towards the nearest ghast.

Attack: 26, Damage: 11

Marrin's whistle has a visible effect on the ghasts, three of them more than the other. Avaria steps forward and the javelin impales one of the ghasts (Ghast 3) caught in the webbing. It slumps forward, still propped up by the web and the shaft of the javelin, but any form of life it had has gone.

Of the three ghasts left the strongest looking one (Ghast 2) is unable to break free of the webbing. The other two though move forward and eventhough they have climb around the webbing and wade through the water, they're able to just reach the shifter.

One of them slashes at Avaria but is still struggling with the water and the affect on its balance and the attack goes wide. The ghast (Ghast 4)that had less distance to cover has more time to adjust and it strikes quickly and hard (24 to hit for 10 slashing damage plus need a DC 10 CON save or paralyzed from whatever nastiness is on those claws).

The group hears another splash behind them, but quick looks reveal Ula has joined the fun.

She pushes her shield forward and a flash of light streaks forward, towards the ghast that just hit Avaria. But Ula should have waited to get her balance and bearings in the water as the bolt of radiant light passes harmless by the ghast.

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Ghast 1 and 4 are in melee with Avaria. Ghast 2 is stuck in the web near the bow still and Ghast 3 is dead.

Avaria will also need two more DC 10 CON Saves from the separate,  but equally foul, stenches of the two ghasts (3 DC10 Con saves in total)

"Great shot Ava, we got these last three no problem!"

Marrin encourages Ava.

((A D6 of bardic for all those saves, if needed!))

She'll then explain to ghast 1,

"Somehow being underwater didn't help clean that stink off of you? Amazing.

Vicious mockery DC 15 Wis save (disadvantage on next attack on fail and): 3 damage

Remembering that these creatures are resistant to her attacks, Ethel still has no other viable option to try and hurt these things.  "Hold on, Ava!" she cries as the makes a fist and summons forth her ghostly, clawed hand.

[Targeting the restrained Ghast in the webbing...]

Attack:  25  Necrotic Damage:  1

((Bardic can be used on the second two saves but not the first - the first is a result of the slash from the ghast - the second two are from starting a turn within 5' of the two))

The insult about the smell appears to be too much for Ghast 1. It looks sadly at Marrin and then just collapses under the water (and removes one of the saves Avaria needs to make).

While Ethel's spectral hand is able to get a good grip on the stuck ghast, the minimal amount of damage isn't able to affect the creature.

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Ghast 1 and 3 are done for.

Ghast 2 is restrained in the web.

Ghast 4 is in melee with Avaria

Avaria and Jan are up.

((Ah, true on the timing, should have done it last turn!))

Marrin blinks at how effective her insult was. It must have been really sensitive about it's stink.

Ethel grins savagely when Marrin drops a second Ghast... but then a curious look passes over her face.

"Ula," she says with concern, "... if you're down here, darling, who is up there holding the rope?"

Jan pulls her crossbow into position and shifts her hex to the one in the webbing in hopes of keeping it webbed a bit longer. 

She looses the bolt at the one on Ava.

Crossbow Attack: 13 Damage: 11 piercing + 2 bludgeoning 

(Restrained one has disadvantage on STR checks from hex.)

Marrin looks rapidly between Ethel, Ula and then the hatch above on hearing Ethel's question.

Ula looks up at the rope hanging limply and shrugs and smiles sheepishly "That's an after the fight problem" she says. "I couldn't just wait up there."

Jan's crossbow strikes home; a dark thick ichor oozes out of the ghasts shoulder as a result. It's clearly seriously injured but still up and ready to fight.

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Avaria's up.

Ghast 2 is restrained in the web - and now suffering under Hex.

Ghast 4 is in melee with Avaria but looks pretty weak.

A limp rope hangs from the hatch - did Ula have time to tie it off, or is it just resting there under the weight of the remaining coil?

((Ooooh the suspense!

Marrin starts looking for alternative ways up that won't require box stacking.))

(CON saves 10, 18, 21!) still have the inspiration, thanks!

Ava hears the conversation about the rope, but decides that is an issue for later.

Growling at the ghast in melee with her, she swings her greatsword at it's neck, hoping to end it for good.

Attack 20, Damage 12

Avaria ends the neck and the ghast in one swoop of the sword, severing the head cleaning from the pale body. It flies through the air and splashes in the water at the feet of the other ghast still stuck in the webbing.

The last remaing ghast strains at the webbing holding it tightly. For a moment it looks like the web will hold but with a last push it starts to strain. Unfortunately for the ghast, he runs out of strength before the web does and it remains restrained for another turn.

Ula, taking her time this go around, again sends a brilliant beam of light at the ghast, lighting it up (Guiding Bolt for 14 radiant damage. The ghast's skin sizzles, but it's still standing.

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Rest of the party is up.

Marrin mocks,

"Smells like someone burning trash."

Vicious mockery (DC 15 Wis save): 2

Ethel brings her spell down upon the retrained Ghast once more.

Attack:  22  Necrotic Damage: 7