Apparently takes place in the same setting as >>3631477:

The black and white griffon makes another swooping dive towards the battlefield, and knocks one of the blank-faced drones from its perch on a rumbling cannon. Such diving attacks are dangerous, but they delay the advancement of the enemy line. At least by a little.

She feels like it's all she can do.

Behind her, the glimmering spire of the crystal empire is within sight. She knows that the young princess Cadence is doing... something... in the castle, but she's not sure it will be enough. It doesn't seem like anything will be enough.

Panting, she beats her broad wings, trying to stay above the swiping claws of a twisted, scaly beast that she thinks was a yak once. But before she can soar away, she sees a green light swelling behind her.

It's all the warning she gets before a wall of green flame wraps around her, gouts of green fire pouring over her back and shoulders. Her beak opens wide in an eagle-like cry and she struggles to stay conscious as she falls towards the ground.

Flapping desperately, she barely manages to slow her decent. She crashes into the ground with a clanging, unnatural sound. Her hot, heavy wings feel like they've been dipped in molten metal. She dazedly turns her head to look at them and sees the tips of her feathers sticking together. She tries to stand, but her entire body feels slow and stiff.

Her head pounds. With a start, she realizes that her beak is hanging open and her tongue feels strange. She remembers opening it to scream as she fell, but can't recall closing it. Her eyes widen, face scrunching, as she finds herself unable to move her jaw.

Her talons suddenly feel as if they are being pulled together, clapping into one another with a dull thud. She looks down to see them grasping at each other, talons interlaced. As she watches, they start to merge, darkening into tough rubber. Her claws blunt and shift, shining like metal. She feels her hind legs start to curl, warping, turning in on themselves. Her tail feels strange, as if it's hollowing out, bulging in the middle.

Somewhere deep inside her chest, the flames she was blasted with seem to have taken hold, flaring and sparking in time with her breath. The heat seems to spread to every part of her. It makes it hard to feel anything else. Hard to think. Hard to exist.

The burning in her core separates into two. One begins to form a churning engine, rattling and pumping a cloud of noxious smoke up her tail pipe. And above it, a pool of liquid fire settles in what were once her lungs and begins bubbling up into her throat. It burns its way up her neck, into the base of her tongue. Her tongue itself now protrudes from her mouth, hardening into hot, hollow metal as her beak sharpens and grows cruel around it.

The fire sears at the base of her skull, burning away scraps of her mind. She faintly feels her frame rock, suspension absorbing a small, insignificant weight, as a PILOT mounts her. Small, rubber-coated hands manipulate parts of her she shouldn't have. Her engine lurches into gear and wheels begin spinning. The sensation of what were once flesh and blood limbs spinning around and around and around is disorienting, but it's almost lost in the burning heat.

She feels like she's slowly cooking in her own mind. Melting down into slag. The fire spills up the tubing in her barrel, waiting for a trigger to be pulled. She feels her head ratchet around to point at something, and knows she's being aimed. The fire will consume all, burning away flesh and will and magic, and leave only hard metal to serve King Sombra.

Fire erupts. A great, burning banner of green scorches the battlefield, spouting from yet another mechanized monstrosity. Several combatants dodge the courting gout, but two pegasi and a hippogriff are caught by the flames. Feathered wings begin to transmute into metal slabs, and a feathered throat budges with the breach mechanism of a piece field artillery.

The war machine moves ever on towards gleaming crystal walls.