She falls into dark waters.

The impact is quite a bit less hard than she might have expected, given how fast she was falling. Instead, the surface of the water stretches rather oddly in the moment that she hits it. The water bends under and around her, as if to cushion her fall in what briefly resembles a large bubble around her... before it collapses a moment later with a soft blorp, immersing her in cold, wet, dark, and strangely sticky water.

…She can’t swim, no, don’t panic -

Freedom of Movement 

Hallana swims up to the surface, and looks around. She’ll cast mage armor while she does so, because dying removes your buffs. Drat, she was trying to keep that Holy Aura, too…

Aivu splashes around until she reaches the surface.

“Hey, wait, where are we? Is this the Abyss? It doesn’t feel like the Abyss.”

She also looks around curiously, pausing to dramatically study her best friend.

”I was gonna be super mad because you died, and you shouldn’t do that, it would make me sad! But we aren’t dead, so I guess it’s okay. Oh! Was this on purpose? Was it a genius plan? You should have told me that!”

The water is slightly more viscous than ordinary water ought to be, but Freedom of Movement easily handles that mess.

Though mostly dark, there are some lights to see by. Stars above, for one... though they're somewhat larger than one might ordinarily expect stars to be.

More substantially, a floating buoy casts a pale greenish light nearby. Moving lights of a similar hue can be seen in many directions; a keen eye can easily identify the nearer of these as being attached to ships. A brighter and somewhat warmer glow can be seen much further in the distance, though a light fog obscures any details from this distance.

In the opposite direction, but quite a bit closer, several brighter flashes are accompanied by a rumbling series of explosions, as one of the ships begins firing its cannons.

Is there anything interesting to sense within sixty feet, if one uses Blindsense?

There are several alarmingly long and sharp spikes in the water; from around 10 to over 30 feet long, but no more than an inch or two wide, and tapering smoothly to a point along their entire length. It'd be a little difficult to see them normally, especially the ones that are slightly submerged instead of floating on the surface.

In the distance, there are some explosions as whatever the ship was shooting at gets hit. It looks like the ship is attempting to bombard a target perched on the side of a mountain in the distance, visible only by the brief flashing of explosions.

...Concerning. Not more than everything else about this place, but still concerning.

"Aivu, it looks like there's a fight over there, want to see what's happening?"

Flying will be more comfortable, Freedom of Movement or no. She'll rise out of the water.

"Sure! Maybe there'll be more friends! If they aren't friends, then I can be really scary at them."

She'll fly after Hallana, although she's doing significantly more loops and small detours, and still keeping up easily.

What's this ship shooting at, exactly?

On closer inspection, the 'mountain' is quite unreasonably spiky, and is potentially the source of those long spikes. As the pair fly closer, the explosions stop, returning it to relative darkness as the gunners reload... until the 'target' suddenly reveals itself as a vast lid opens. It's a single colossal eye, nestled among innumerable sharp spines, whose iris glows with a strange and eerie dark light of a colour the two have never seen before. The eye glares angrily in the general direction of the ship (and, incidentally, the adventurers).

There is a brief moment of terrible stillness, before the entire 'mountain' screams out its hate. The scream is louder than anything you can remember hearing, and in its wake, the skies begin to fill with tongues of flame, which immediately begin to form into complex-looking sigils that are at once song and grief and law and fury. The waters boil around it. The metal hull of the offending ship begins to melt. The world itself seems to bend... and the terrible thing begins to move, faster than anything its size ought to be able to move, charging towards the ship that attacked it.

Colors she’s never seen before is not actually an unprecedented experience, for Hallana, but it’s still weird. And-

Ow. Her eardrums are already healing, but that was rude. And the water’s hot enough she’s going to cast protection spells for Aivu; her personal immunity doesn’t transfer. She’ll quicken them, this seems time sensitive.

Hm. The attack was likely not aimed at her, and she still doesn’t know the situation enough to feel comfortable interfering. She’ll get closer, to see if the not-mountain’s amenable to talking.

Actually, that's exactly what it feels like doing. But, as any Zailor might tell you, Lorn-Flukes only understand sorrow and anger... and though they breathe water, they speak fire. This one is speaking quite a lot of it, and the twisting burning sigils seem like they might almost make sense if you stare at them long enough...

Meanwhile, the offending ship is visibly melting even as it fires off a second barrage, its shots breaking off dozens of enormous spines, but somehow managing to miss the great eye. You're close enough to see more details; on the half-molten deck of the ship, a hulking figure in strange brass armour braces against the oncoming charge... and as the Lorn-Fluke violently collides with their ship, driving hundreds of sharp spines into the vessel, they leap onto the thing, delicately climbing the mass of deadly sharp spines one-handed while carrying what looks like a simple bone spear in their other hand.

There's something there, maybe, but she can't actually understand it. Human sailors it is! She'll see if she can get some goodwill by casting a communal protection from energy (fire) in a 25-foot radius from the most visibly molten part of the ship that still has sailors on it.

Healing would be better, really, but she doesn't have any. 

"Hello, folks! What's happening over here?" Their answers might even be informative!

The sudden arrival of a flying person would be rather surprising... if it was even happening at all, which seems unlikely. It must just be some strange apparition, either caused by the Fluke, or attracted by it. And there are far more immediate problems at hand.

"Engines full astern," yells the zailor at the tiller, with the urgency of one who knows they don't have long to do a dozen things exactly right, as he gives orders to his fellows, "Seal the bow 'atches! Man the bilge pumps! An' cut loose some o' them b____y spines before the d__n thing pulls us under! Cap'n's off seekin' 'is death again, so it's well past time for us to get well gone!"

Given the combination of mortal peril and clear orders, the other zailors quickly obey, as opposed to trying to talk with the possibly-hallucinatory flying person, though a few of them are at least confused with why it suddenly doesn't seem to be as hot.

Well. That’s rude. Probably reasonable of them, though. Unfortunately, she doesn’t have many options have options that would actually help, here.

She’ll cast a light spell on herself, to make her more obvious, land on the ship, and try asking the closest sailor. If that doesn’t work, she’ll try to Dominate them; it’s unfriendly, but she really wants to know what’s going on,

A glowing person who is actually walking around on the ship's deck is somewhat less absurd; there's lots of weird glowing things in the Neath. One of the zailors is busy frantically working a saw back and forth across a long spine where it sticks into the ship's hull, but he spares a few words of reply. "Wot's 'appening? B____y murd'rous fluke's 'bout'ta drown us!"

There is a small explosion from below, and the deck lurches and tilts in a disconcerting way as the ship pulls partly free of the fluke. Unfortunately, it's not doing so in a very even way; said fluke has shut its eye again and is submerging in a leisurely fashion, while the ship is listing quite a bit where the spines are pulling and tearing at the hull. A sharp broken spine falls from above and impales a screaming zailor to the deck.

Damn it all. Whatever, worst case, one person’s easier to resurrect than a ship of them. Protection from Arrows for everyone in a 30-foot radius, and then Haste, extended from a rod. It’ll only last for maybe four minutes, but that’s basically eternity in combat. She can’t do much to keep a ship from sinking, but being faster will probably make the sailors better at it. She’ll indicate that Aivu should come over if it’s reasonably safe, the hot metal hasn’t given the sailors too many problems, so it’s probably fine for a dragon.

Is the captain still on the fluke? What does he seem to be doing?

No part of this mess is particularly safe, but the deck is more deformed and listing than actively molten at the moment, as the initial bursts of fire and heat have died down. A few more spines fall, though the zailors closer to the bow are being more careful after seeing their fellow impaled to the metal deck though his screaming quickly gave way to vomiting and convulsions. The haste is a little more surprising, but everyone seems much too busy to question it right at the moment as they desperately try to pry the ship loose before it capsizes.

With the chaotic motion of the ship as it tries to break loose, and the forest of sharp spines jutting out from the fluke in every direction, it's hard to see just where the captain is... but taking a few moments to looking carefully, Hallana manages to spot the armoured figure at the corner of an eye much larger than they are, apparently trying to drive that bone spear up in between two massive eyelids.

Oh no! Aivu’s going to pry the spike off the sailor, and cast Cure Moderate Wounds on him! 

Hallana, meanwhile, is going to see if a massive bolt of holy light (24d6 of holy damage as a touch attack) does anything significant to the fluke, and will fly closer to the captain while doing so.

Cure Moderate Wounds is sufficient to keep the impaled Zailor from the River for the moment. Further medical care may need to wait until the vessel is stable enough for the spine to safely be removed.

As for Hallana, she finds the the Lorn-Fluke to be an exorbitantly large target. There's certainly no question about hitting the creature; it's singular eye is larger than the targeting range of that attack... though it is closed. The captain is in up to his elbow trying to push their bone spear between eyelids, menaced by the foot-long spikes that serve the thing for 'eyelashes'. Getting that close seems a bit too reckless, but anywhere in range is certainly much closer to the captain than not. The bolt of light she unleashes doesn't quite hit right between the eyelids, but it hardly matters; it's powerful enough to burn through the flesh and pierce through to the eye beneath. It was certainly not ineffective; the white of the Fluke's eye is now clearly marred with a two-foot burn hole, which is starting to ooze a liquid that seems darker than black.

As the bolt of light fades, the Fluke halts its dive and opens its great eye, to more closely observe whatever it was that did that... and the tangible weight of its regard falls upon an improbably flying figure, hovering right in front of its eye. For the moment, Hallana has the Lorn-Fluke's full attention.

“I’m willing to drop this, if you’ll leave without harming anyone more!” she shouts, while flying closer.

She’s also casting Shield on herself, because she isn’t feeling particularly hopeful today. And Quickened Mage Armor.

Aivu’s very strong! Can she help with the prying?

No one seems to be busy trying to unpin the injured zailor at the moment; instead, they're focused on is trying to get any spine that is both embedded in the ship and connected to the Fluke free. Much of that involves sawing and prying, some of it by zailors haphazardly hanging off the bow of the ship on ropes, working to access more problematic spines.