Seventh day of November, 1740. My ancestor Richard Blackthorne made a bargain with something in the spaces between things. He was a man of science — an alchemist, though he would have called himself a natural philosopher — and he spent his nights in the stone tower on the Galway estate, running experiments with lenses and mirrors and distilled minerals until he found the recipe that would open the door.

The recipe was simple: a vial of laudanum, a silver bowl polished to mirror brightness, and a poem in Latin that nobody could translate. Richard drank the laudanum, gazed into the silver bowl, and recited the poem. The tower door sealed itself from the outside. He did not come out for three days.

When he did emerge, he was a different man. Older, somehow, though only seventy-two hours had passed. His eyes were flat and empty, like a photograph of a man rather than the man himself. He wrote one sentence in the family journal and then spent the rest of his life sitting in a chair by the fire, staring at nothing.

The sentence read: "The universe needs watchers. I am one of them now."

That was three hundred and eighty-five years ago. Since then, one member of the Blackthorne family has been sent to the watch every generation. They are chosen at twenty-one years of age. They are given a vial of laudanum, a leather-bound journal, and a single instruction: write everything you see.

No one explains what "the watch" means. The family simply prepares the initiate with the quiet solemnity of a religious ceremony. The current Keeper — currently my aunt, Lady Isolde — escorts the initiate to the tower. The door is sealed with iron bolts from the outside. The initiate enters. There is no body recovery.

I always thought it was a metaphor. A poetic way of describing solitary confinement, or penance, or some ritual I didn't understand.

I was wrong.

Dr. Alistair Croft, the family physician, came to see me yesterday afternoon. He is a small man with large eyes and a manner that suggests he knows things he should not know. He sat in my study, poured two glasses of brandy, and told me the truth.

"Your family is not a family," he said. "It is a machine. A very old machine, built three and a half centuries ago by your ancestor Richard, who made a bargain with an entity from between dimensions. In exchange for longevity, wealth, and protection, your family provides one human consciousness every seventy-five years. The Sentinel does not die. The Sentinel is relocated to a place that exists outside normal spacetime, where they serve as a living observer."

"Why?" I asked.

"Because the universe requires observers to collapse quantum waves into reality. Without enough observers, reality becomes unstable. Your Sentinel exists in the space between dimensions, observing the universe continuously, keeping it real. Richard Blackthorne was the first. I am sitting in his chair right now, and I have been for seventy-two hours."

I laughed. I could not help myself. "You're telling me that our family — the Blackthornes, who have been producing mildly interesting poets and mildly interesting scientists and one absolutely useless heir in my case — that our family is responsible for keeping the universe real?"

"Not your family," he said gently. "You. The Sentinel. Your family provides the vessel. You provide the consciousness."

He stood up. He put a vial of laudanum on the desk. He put a leather-bound journal next to it. He looked at me with an expression I can only describe as pity mixed with awe.

"The tower is ready," he said. "It will be tonight."

I did not sleep. I walked the estate grounds in the moonlight. I stood before the tower — a stone structure built in 1680, ten feet in diameter, with a heavy iron door that has not been opened from the inside since Richard Blackthorne emerged from it three centuries ago.

I am writing this diary now, in my study, by the light of a single candle. The laudanum sits on the desk. The journal sits beside it. I am twenty-two years old. I am engaged to a woman I do not love. I have never accomplished anything of note. I have published one mediocre poem in a student magazine. I suffer from melancholia, according to the family physician, which is a polite way of saying I am depressed for no identifiable reason.

I think I know why now.

I am not afraid. I am fascinated. I understand, in the way that a man understands something before he steps off a cliff, that I am about to discover the mechanism by which reality exists. I am about to become, in a very literal sense, the eyes through which the universe sees itself.

But I am also about to be alone for the rest of eternity.

There is no rescue. There is no return. The Sentinel does not come back. Richard Blackthorne came back once, in 1743, but he was not Richard anymore. He was a shell, a husk, a man who had seen too much and could no longer pretend that the world was ordinary.

I will write this diary so that someone, someday, will know the truth. The universe is dark. Not empty — dark. Full of watchers, yes, but watchers in the dark, observing from places between places, and the darkness is not just the absence of light. The darkness is the presence of something vast and indifferent that we cannot perceive because we are too small and too short-lived and too trapped in our brief human lives to see it.

Light is dangerous. Not morally dangerous — physically dangerous. To observe is to collapse. To collapse a wave function is to choose one reality over infinite others. Every time someone looks at the stars, they are killing infinite possibilities and choosing one. The universe is dark because light kills. And we are all lights, burning through our brief lives, killing possibilities left and right, creating reality by looking at it, and the watchers in between dimensions are the only ones who see the cost.

I am going to the tower now. The door is closing. The iron bolts are sliding home. I can hear my aunt in the courtyard below, praying in Latin, the same Latin that Richard used three centuries ago, the same Latin that Dr. Croft said was not a language but a frequency.

If anyone finds this diary: do not look for me. Do not try to open the door. The universe is safer when we stay in the dark.

Goodbye. I am Sebastian Blackthorne, and I am going to keep the universe real.

I hope it is enough.