The Heftbound are what the Implosion breach made of the dwarves who stood closest when the Dynus seam tore. Force came loose in the same instant their living ended, and in some of them it did not pass through and away the way shaped force passes through a force-worker. It lodged. They are not undead. No necromancer reads a shadow-trace on a Heftbound, and no road runs down to Malstaris beneath one. They are not alive either. The breach caught them between the two states and left them there, animate and heavy and not quite either thing. Twenty thousand years on, they still move through the crater that killed the rest of their city.
What the force took
A living person is carried by three threads. Two make up the life, the soul that rises to the Astral when the body fails and the shadow that goes down to Malstaris. The third is the spirit, the self and the name, which ascends to Celestia and endures only while the name is still spoken. Force, the Dynus element, the bare push under any wanting, worked into that set when the seam broke and took the place of part of it. Where a soul or a shadow should sit there is raw kinetic force instead, drive without anything driving it, and force will not do the work those threads were meant for. In some the force lodged in the spirit, so the will that moves the body is itself a thing of thrust and weight. In others it displaced part of the life-pair. The wrongness is the same either way. The threads that should carry a Heftbound out of life are tangled with an element that answers Gaea's bridge and not death, and the body cannot finish the passage those threads were for. So it holds together, and moves, and bends the ground as it goes.
What the surveyors named them
Surveyors who mapped the crater after the Collapse sorted the Heftbound into two kinds by what the force in them does. They are the same being either way, parted only by which direction the lodged force pushes.
Crushers are the ones the force drives outward. They fall on anything warm with a weight far past what a body their size should carry, a single downward blow that cracks stone and flattens what it lands on. They do not strike twice. They do not need to.
Gravity Wells are the ones the force turned inward. They drift through the ruin in slow circuits, and the pull of the ground bends toward them as they pass, so that loose rock, spilled tools, and careless climbers slide across a level floor toward the thing at the center of the lean. A Gravity Well does not chase. It waits, and the floor does the work.
What they reach for
Left alone, the Heftbound move toward the living the way the Cinderbound of the burned country move toward heat, slowly and without obvious aim, drawn to anything warm and moving. Under the pull sits the older thing the breach made. A death that was never allowed to finish. The force welded into them carries Dynus's single quality, the urge to keep moving, to spend the thrust it holds, and that urge tugs them at whatever lives. They cannot spend it. The thread that should carry them down is busy holding force instead. So they fall, or they pull, and the want is no nearer answered than before, and they drift on through the crater to the next warm thing.
When one is destroyed
A Heftbound can be put down. Break the body apart, or wear it down until the shape it held comes undone, and it stops. But the moment it stops is not quiet, and the difference between a Heftbound's ending and the other bound things' is the whole of the warning.
Set it against what the elements leave behind. A destroyed Cinderbound stays where it fell, a patch of fuelless heat fixed to one threshold. The bound things of the high open air scatter on the wind and haunt no place at all. An Hourbound holds its place and comes loose from when, walking a road three winters late. A frostwalker's roads freeze where it falls, sealed and silent. A Heftbound does none of these. The force lodged in its threads cannot stay once the body that shaped it is broken, and it cannot linger or scatter or wait. It comes out all at once. The kinetic charge that stood in for a soul or a shadow discharges in a single concussive flash, a soundless blow of pure force that throws everything near it off its feet and cracks the ground where the body fell, spent in an instant and gone. Only after that, in the stillness behind the blast, do the threads it leaves take their partial roads, late and incomplete, the way a compromised strand-set always ends. There is no remnant to haunt the place. The danger was never what lingers. The danger is the going off.
Laying them to rest
This is the one mercy in the Heftbound and the one cruelty, and they are the same fact. Destroying a Heftbound does lay it to rest, after a fashion the other bound things are denied. The force that held its threads hostage is released in the discharge, and the freed threads, partial and mistimed though their roads run, are at least let go. No fire-haunt is left, no scattered wind, no figure walking late. But the release is a blast that can kill whoever delivered the killing blow, and most who go into the crater to put a Heftbound down are not braced for a body to answer its own destruction with a concussion. The dwarves who understood the seam might have drained the lodged force back into it before breaking the body, the way a Cinderbound can be drained at the World Fire seam. The dwarves are gone, and the seam at the crater's heart lies under the deepest reach of the inward pull, where no one has reached. So the Heftbound are destroyed, and the destroyers brace, and that is the nearest thing to rest the crater allows.
Hooks
The braced crew. A salvage company means to clear the crater's upper slope of Crushers to reach the dwarven workings below. They have lost two crews to the discharge already, having sold the work as ordinary monster-clearing and not as the demolition it actually is. They are hiring a third, and still not saying so.
The line that holds. A survey rope lowered years ago lies flat against the inner slope and will not be pulled up, as though something at the bottom has the end. The family that lost the surveyor wants the body recovered. What holds the line is a Gravity Well, and the bottom of the crater is the master regulator, and reaching either means crossing the full weight of the inward pull.
The well that learned the path. A single Gravity Well has drifted the same slow circuit through the ruin for so long that the floor has worn a groove toward it, and caravans skirting the crater's rim have begun to lose animals over the edge on calm nights. The thing has not left the crater. The pull, it seems, is reaching past the rim, and no one can say whether the well is wandering or the crater is widening.