Where the Underworld extended under a lake or river, water had flowed into the cracks in summer, and turned to ice in winter, year after year. Where gremlins, dark elves, or worse maintained the tunnels, that was as far as it went.
But in a long-disused tunnel, a crack opened clear through in winter began to release its payload in the spring thaw, water dripping to the tunnel floor… and at an opportune moment, releasing an ice crystal to clatter and splash into the puddle.
With time and symbolism united, the crystal grew, sucking up the puddle as enthalpy was pumped into it involuntarily, and catching each new drop with an instinctive hunger.
Once it was big enough to do anything voluntarily, it formed itself into a tall spike, reaching up to the ceiling, into the crack, and touching the ice there… rapidly claiming the entire web of ice as its own body. It flowed down, under shapeshifting magic rather than proper fluidity, into a seventy-grav heap on the tunnel floor, writhing there as it shaped its body into a demonic humanoid form, then cleaning up the details.
She paused there on her hands and knees, panting, heart thundering, until she had recovered from incarnating.
She stood, and carefully tested her breath before speaking, to make sure she could deliver the thoughtful purr she needed to distinguish the halves of her mind. "Well, Ride darling, that was rougher than most, but we made it through."
Then as the other half she replied, distinguished by a self-indulgent dry humor. "Lysia darling, I've been through worse deaths, and I'm confident we've been through worse deaths together."
"True enough," she purred, "but that fire was designed to destroy even your people, and I'm all but certain it has a chance. Past performance and all that."
"True enough." she replied drily.
A pause, and she turned to her tasks; Ride reached up to form a mirror of ice on the wall, while Lysia poured a bit of mana into a floor tile through her feet. Ride was a devil, far greater than but similar in kind to the gremlins normally used to claim these tiles, and their combined body had the power to do the same, leaving a little violet spurt of mana returning. More would come; she'd have to collect it manually until she got the place properly wired, but even one claimed tile meant she wasn't spending a fixed supply.
Lysia indulged Ride's hungry gaze, but mentally skipped over the parts Ride focused closest on, as they had gotten used to. She looked almost human, with broad hips and shoulders, pure white skin — a bit of color would come in time — and the appearance of a bustier, made of her silver-blue scales. Scales also formed "gloves" from her elbows to the clawed tips of her long fingers, "boots" from her knees to the talons of her prodigious digitigrade feet, and something like a forehead tiara leading to considerable horns on each side. Above that her hair was a generous plume of deep blue, which flowed to mid-back, where her flexible spine led down to a scaly, muscular tail.
Her face was a long-worn compromise. An elfin chin, full lips(in need of color), high cheekbones with a dusting of scales, a slender nose, big almond eyes with icy irises and the lids for wonderfully-fine control when narrowed… beautiful and terrible, as if she were fit to rule, despite the lack of subjects.
Ride raised a hand as if wrapping one finger at a time around a pole, and held it there until Lysia yielded and spent the mana to actually form their scythe, from conjured ice.
"We really should conserve mana until we find a bit of orichalcum." she purred.
"Badge of office."
"No office without the mana for gremlins."
"Is that a challenge?"
She sighed promptly, as Lysia anticipated the usual conversation. It wasn't a sound that clearly came from one side or the other, so she contemplated the question for a few seconds. "Nnah." She glanced back and forth down the tunnel, and Lysia started in one direction, claiming the next tile.
"Of course, whichever way we pick will be wrong. And once we get the place wired, it won't matter that we collected the mana by walking back this way."
"Irony everywhere one looks." she purred.
The first way dead-ended shortly after shifting to harder, mountainous rock; the second led to an intersection, still under the lake, then off into some dank network. That she had to double back again was either more irony(Ride's opinion) or less irony(Lysia's).