A Sentry is a tall armoured figure, half again a man’s height, made of living bronze and temporal energy. Its plates have the deep brown-gold of old bronze, with the patina of great age though nothing in the Lock has aged, and through the seams shows the faint, slow-pulsing gold of the vault’s own light. Its face is a featureless mask of polished metal, smooth as a mirror; a visitor sees their own face in it, stretched.

It does not move unless it must. When it does, the motion is excruciatingly slow, a raised hand taking what feels to a visitor like an hour, and it carries immense force; it seems less to move than to arrive. Its armour chimes, a low, resonant note that rolls through the Lock’s thick air and reaches the ear long after it was struck, over the deep thrum of the vault. The air near a Sentry smells of metal and of the faint electric scent of temporal energy.

Bronze Sentries stand within the Lock, among its massive walls, iron doors, sealed alcoves, and shelves of things wrapped in cloth, sealed in resin, or cased in crystal, on a floor of interlocking plates engraved with sigils of time. Each Lock has its own; how many, nobody outside has counted. They do not leave. They are immortal as long as the Lock stands.

The Bronze Sentries hold their duty sacred and will not compromise it. They value discipline, patience, and refusal, and they are silent and implacable. They perceive time at the Lock’s rate, so to them the world outside is a blur of incomprehensible speed, and a visitor is something fast and reckless that has come in from the blur. They speak through gestures that take hours to complete and through the chiming of their armour, and a visitor who wants an answer must wait for it, at a cost of ages outside.

They are not cruel, and they are not moved. A Sentry does not bargain with the Interred, the beings sealed in its keeping, however long they plead, threaten, or deceive; it does not bargain with visitors either.

A Sentry does not eat, sleep, or breathe; it draws its sustenance from the Lock’s temporal energy and endures as long as the vault does. Its slowness is the Lock’s, and so is its strength: what a Sentry’s hand closes on cannot be pulled free. Above all it can seal. A Sentry that judges a visitor a threat may seal them inside the Lock, and once sealed there is no appeal; whatever is sealed stays as it was, for as long as the Lock holds.

The Bronze Sentries are why the Lock is trusted. People come to lodge a dangerous object, to retrieve a sealed one, or to consult the Bronze Sentries about a threat nothing else can hold, and the Bronze Sentries are the only consent that opens the vault. The dangers are the Bronze Sentries' judgement, which a visitor cannot appeal, and the visit itself: the shortest audience costs ages outside, and those who come out find their world has long forgotten them.

People believe the Bronze Sentries are the vault’s first builders, who stayed to guard what they had sealed; or angels set by a god of time; or the bronze ghosts of those who sealed themselves in with their burden. Who built the first Time Lock is not known, only that whoever did understood that some things are too dangerous to destroy. Some sealed temples keep a legend that their Sentry will one day finish a gesture it began when the temple was young, and that the gesture is a warning.

Thieves' songs tell of the master thief who walked into a Time Lock for one sealed crown, was out again in a dozen heartbeats, and came home to find his guild, his city, and his language gone.