Well fuck.

If he systematically goes through the internals of its body, starting from the head and working over every cubic meter, trying to turn it to hot air?

He does not seem to be able to actually affect anything on or in the monster (coral excepted).

Well that is fucking awful and kinda the only way he has to damage it, he thinks. He doesn't really have any way to protect people from the weird dust forcefield, only he is indestructible and he doesn't have a convenient way to extend it to others.

Time to try brute force with his indestructibility, perhaps.

He flies towards it.

It does not react to the tiny winged humanoid less than a hundredth of its volume flying towards it.

The Al Bhed charge their machina.

Theo can make quite a sharp blade. He is not averse to making a sharp blade that is attached to him and technically part of him and therefore less breakable.

So he gets close, his tiny self with his very low volume and his very sharp blades that are indestructible, intending to start carving a small hole through Sin in case this distracts it.

Theo can go through the forcefield. His clothes can't. It feels like electricity and fire and hurt and it damages him exactly to the extent he can be damaged.

When he makes contact, he causes more damage than Sin has ever suffered at the hands of anything other than aeons. Inasmuch as it can react, it does, by turning its body slightly towards Theo and trying to swat him away with one of its huge fins.

He grits his teeth as he goes through the forcefield, and he is unfortunately able to be swatted. He tries to damage the fin as he is so, then goes quite a distance quite dazed but not going unconscious and trying to get himself under control and then he will be on his way back.

He makes the blade bigger. It doesn't need to be particularly heavy if it's really sharp and indestructible, though he doesn't make it too low density because he wants to still have momentum.

It can get really sharp, when indestructible.

That it can. The second time he makes contact, Sin reacts quite violently, turning its whole body much more quickly than it by rights ought to be able to, probably swatting Theo away again. Its purple forcefield winks out, and the black rippling one enveloping its body returns.

Again, he can't screw with his velocity directly, though he can – once less dazed – try to screw around with his environment to speed up his recovery.

It is really painful getting swatted, but he's on his way back because at least he has a limit to how damaged he can get here, and he's apparently getting a reaction.

He is!

The black forcefield does not burn him, though. It is in fact pretty solid—much more solid than Sin itself.

Not that Theo can't cut it, it's just—less immediate. There's some resistance.

Even with the really sharp blade? It is really sharp, he can keep making it sharper, he does keep making it sharper to counteract wear and tear because despite his indestructibility it is not totally invulnerable and can be weakened; he is actively changing it and maintaining the edge as he cuts.

The black forcefield is apparently doing something other than just "being durable" because no matter how sharp he makes the blade it has the same resistance.

Sin generates a purple sphere of energy in front of its "mouth" and shoots an energy beam towards the big (the meaning of the word 'huge' will probably never be the same for Theo again) Sinspawn in the cage, turning it and its cage to dust. Then it starts actually turning away.

Theo manages to cut through the forcefield at the same time the Al Bhed machina finishes charging, so now the monster is being hurt on two different fronts.

He keeps going, trying to force his own way through the forcefield if he can, too – if he can grab onto it, if he can slice a hole and it doesn't just reappear, if he can insert himself in the way then hopefully it won't be able to regenerate and he'll be able to cut even more of it.

It takes a while to regenerate, and he can get through the field before it does.

Sin finishes turning and starts accelerating away and into the ocean, suffering grievous wounds from the machina and an angel that won't. Stay. Gone.

The angel conveniently has a blade stuck in the huge creature. And is attacking the forcefield from being inserted through it which is much more convenient and hopefully he's able to stick attached to the freaking thing.

The freaking thing is starting to move really fast.

Underwater.

… Theo doesn't need to breathe.

Water resistance at over a hundred kilometres per hour might be a problem, though.

He's past the forcefield and digging his way in so he's not totally in the path of the water. It might be, but he has a hold on Sin with his blades and is digging in.

Plus he can crouch in, make himself flatter against Sin and hold on better, being annoyingly indestructible and getting out of the path of the water resistance.

Yep, that he can do.

And after digging in enough, the resistance is suddenly gone. As if Sin is—hollow.

– Okay he has no idea why that'd be but he wants to know what is inside if not more of Sin, so he keeps digging to enlarge the hole and narrows himself down somewhat to get in.

Sky.

It's the first thing he notices. The inside of Sin contains the sky. Clouds, beautiful clouds, blue skies, and pyreflies, flying in all directions.

The inside is definitely bigger than the outside.

… He's really not used to this magic system yet.

Is there anything like civilization? Presumably there is a ground. Is he somehow back outside Sin just a bit displaced, does the sky have a sun, is it raining anywhere in sight, can he still see walls or have they disappeared, what.

There isn't anything like a sun—just a vague glow present everywhere, possibly coming from the pyreflies. There is not a ground anywhere that he can see, but the clouds are occluding enough that that doesn't mean much—there is a carpet of them extending in all directions below him.

Behind him, there is a glowing ball of light, marking the exact spot through which he came. It's not embedded on anything, certainly nothing like a wall, and in fact walls don't seem to be present at all, either.

He can put his hand over to the side of the glowing ball? How big is it, can he put his arm around it? Because. That's weird.