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 imps, 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, heart thundering, desperate breaths washing through her like ocean waves, until she had recovered from incarnating.
She stood, and carefully tested her breath before speaking, to make sure she could speak in 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 an 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 kind, and I'm all but certain it has a chance. Past performance and all that."
"True enough." she replied drily.
A pause, and they both turned to their 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 to the imps Lysia used to claim these tiles, and their combined body had the power to do the same, leaving a little blue spurt of mana returning. More would come; she'd have to collect it manually until she got a Heart together, but even one claimed tile meant they weren't spending a fixed supply.
Lysia indulged Ride's gaze, as they had become accustomed to. She looked almost human, with broad hips and shoulders, pure white skin — a bit of color would come with a Heart — and the appearance of a bustier, made of her shimmering aqua scales. Scales also formed "gloves" from her elbows to the clawed tips of her long fingers, "boots" from her knees to the talons her prodigious digitigrade feet, and something like a forehead tiara. Above that her hair was a considerable plume of deep aqua, which flowed to mid-back.
Her face was the least-favorite feature of both of them, for opposite reasons; with a hawkish lankiness, she was neither beautiful nor terrible, and the seemingly-random scattering of scales they could never agree on except by silent tolerance.
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 weapon, from conjured ice.
"We really should save mana until we pick a place and get a Heart up." she purred.
"Badge of office."
"No office without the mana to summon imps."
"Is that a challenge?"
She sighed promptly, as Lysia anticipated the usual conversation. It wasn't exactly a sound that clearly came from one side or the other, so she contemplated the question for a few seconds. "Nah." 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 place a Heart, it won't matter that we collected the mana by walking back this way."
"Irony everywhere one looks." she purred.