An elemental knight is a suit of heavy war-armor with a fragment of an elemental plane sealed inside it. The armor is the body; the element is what moves it. There is one kind for each of the nine planes, named for the substance it carries. A fire knight holds a churning mass of Yolus-fire. A stone knight holds a slow grinding weight of Golus that never quite stops settling. They were made, not born, and none has been built in living memory. The ones that remain have been walking far longer than any kingdom now standing.
A living thing needs both halves of its life-force, soul and shadow, with a spirit between them to call itself "I." An elemental knight has none of the three. It is element and binding and nothing else. By any honest reckoning it is not alive, but it is plainly not dead, and it is not undead the way a risen corpse is, because there was never a corpse and there is no road to Malstaris beneath it. It is animate. That is the most that can be said of it.
Appearance
The armor is old and overbuilt, sized for a frame larger than a man's and plated thickest at the joints, where the binding-work runs deepest. Every knight is sealed. There is no gap at the visor, no seam at the gorget, nothing the thing inside could leak through. What gives it away is the light. The element shows at the joins and the breathing-slots as a steady glow the color of its plane, and the metal nearest the core runs hot or cold or worse to the touch.
A fire knight is the easiest to read. It glows at the seams like a forge banked too high, the plate over its chest stays hot enough to scar a hand through a glove, and it roars faintly even standing still, the sound of a fire that wants more air. Snow does not settle on it. Rain comes off it as steam. At night you see one coming a long way off.
Nature
The binding does two jobs at once. It holds the element in a coherent shape, and it presses on that element a shape of purpose, enough to march, hold a line, take an order and keep it. That borrowed purpose is not a self. The knight acts with intent but has no spirit to own the intent, so it wears the outline of a will it does not actually possess. For one campaign that hardly shows. Across centuries it curdles. With no self to anchor it and no death to end it, the knight's borrowed coherence frays, and what is left reads as madness.
In a fire knight the madness runs in Fire's own key, hunger and rage and the urge to grow past every limit, so the oldest of them are simply furious, without pause and without object. They cannot age and cannot tire; the element renews itself as long as the armor holds. Left alone, a knight does not wind down. It keeps its last order until the order stops making sense, and then it keeps some wreck of the order anyway.
One thing every knight still tracks cleanly is the people who made it. The binding was war-mage work, and a knight knows a channeler on sight. This is the root of a hatred that outlasts everything else in them. When the wars they were built for ended, their makers did not release them. A binding like this has a closing word, and the binders carried theirs to the grave or simply walked away without speaking it, leaving the knights sealed, awake, with no way out of their own armor and no maker willing to give them one. What survives of their minds remembers that. A fire knight that has ignored a village for a decade will cross a valley to reach a hedge-mage. The hatred is not for magic itself. It is for the kind of hand that bound them and left them standing.
Origin
The knights were built as soldiers for an age of war old enough that its battles carry no agreed names now. They were not summoned one at a time by hopeful conjurers. They were manufactured, in numbers, by binders who drew a fragment off an elemental plane and worked it into a prepared suit of armor through a craft no living tradition has reproduced. The work was a war-secret, guarded inside the binders' own guild-states and never written down for outsiders. When those states fell, the craft fell with them. That is why no one makes elemental knights anymore, and why the ones still walking cannot be unmade by anyone who finds them, only broken.
What the binders built well, they built to last past the war it was for. They had no incentive to install an easy ending, and several incentives not to, since a soldier that cannot be turned off cannot be turned against you either. The knights were the cheap, expendable front rank of armies that thought the war would end before the constructs did. The war ended. The constructs did not.
Combat
A knight fights with the drill worked into it, holding formation habits centuries after the formation rotted away. The real danger is never the swordwork. It is the element. A fire knight sets light to everything it touches and presses an attack far past the point where a living soldier would break off, because the rage that drives it has no instinct for self-preservation and nothing left to preserve. Cold, walls, numbers, pain mean nothing to it.
You do not kill an elemental knight, because nothing in it was ever alive to kill. You break the binding. Crack the armor badly enough that the seal fails, and the element loses the one thing holding it in shape. It goes home. The fire gutters back toward Yolus the way a draft pulls a flame off a wick, and in a breath there is nothing left but cooling, empty plate. No body. No remnant. No haunt, and no name to be forgotten, because there was never a name in it. A destroyed knight leaves less behind than a dead man does. A dead man at least leaves the three roads his strands take. The knight leaves an empty suit and a little scorched ground.
The plate of a fire knight, an hour after it fell. The breastplate is split along a binding-seam, the cut edges still ticking as they cool. Inside there is nothing: no ash, no slag, no scorched bone, only the smell of a snuffed lamp and metal too hot to lift. Whatever marched in that armor for nine centuries went back where it came from and left not even a stain on the lining.
Hooks
The standing order. A fire knight has held a stretch of empty moor for as long as anyone remembers, attacking nothing that keeps to the old road and burning anything that leaves it. It is guarding a position in a war that ended before the present kingdoms were founded. Someone has started quarrying just off the road, and the knight has noticed.
The closing word. A scholar has reconstructed part of a binder's closing-word from the sigils on a dormant knight's armor, enough to believe the full word could dismiss the thing cleanly, element home and no fight. She needs the knight to stay dormant long enough to finish, and she needs someone standing between her and it if she is wrong.
The wrong target. A fire knight has left its old ground and is crossing settled country in a straight line, ignoring every village and every soldier in its path. It is walking toward one person. The party is escorting that person, and no one has told them why a four-hundred-year-old war-construct wants a particular minor wizard dead.