Merlin created it

We had reached a small chamber, the floor and walls lined in the same stone used in the lighthouse and cottage. I put a hand on them and could feel them quivering with power, with the dissonance of conflicting energies. Actually, looking back, I saw that at some point, the walls of the stairway, and the stairs themselves, had begun to be constructed of the same material—every single stone of it invested with impossible amounts of power and skill.
“Uh,” I said. “Question?”
Demonreach had stopped at a stone doorway shape in the wall. It was surrounded with larger stones covered in intricate carving. The burning eyes turned toward me.
“Um. Who made this place?”
It didn’t speak. It pointed to the door. I looked. There was a sign in the middle of it, a sigil. It wasn’t something I recognized as part of any set of runes that I knew, but I knew I’d seen it somewhere before. It took me several seconds of sorting through memories to run it down—I’d seen it on the spine of a very, very, very old journal on my mentor’s bookshelf.
“Merlin,” I said quietly. “That’s whose sign that is, isn’t it?”
Demonreach did not respond. Why say YES when silence will do?
I swallowed. The original Merlin was the real deal, Arthur and Excalibur and everything. Merlin had, according to legend, created the White Council of Wizards from the chaos of the fall of the Roman Empire. He plunged into the flames of the burning Library of Alexandria to save the most critical texts, helped engineer the Catholic Church as a vessel to preserve knowledge and culture during Europe’s Dark Ages, and leapt tall cathedrals in a single bound. There were endless stories about Merlin. Popular theory among contemporary wizards was that they were more apocryphal than accurate. Hell, I’d always figured it that way, too.
But staring around at this place, I suddenly felt less sure.

Cold Days Chapter 16, Page 154-155


The image sank down until it showed a familiar landing point, though it had no ruined town and no Whatsup Dock and no row of wooden piles in the water. It was just a little beach of dirt and sand and heavy, brooding forest growth.
Then a ribbon of light maybe eight feet long split the air vertically. The light broadened until it was maybe three feet wide, and then a figure appeared through it. I recognized the signs—someone had opened a Way, a passage from the Nevernever to the island. The figure emerged, made a gesture with one hand, and the Way closed behind it.
It was a man, fairly tall, fairly lean. He wore ragged clothing in many shades of grey. His grey cloak had a deep hood on it, and it shadowed his features, except for the tip of his nose and a short grey-white beard covering a rather pointy chin.
Letters appeared at the bottom of the screen. They read: (MERLIN.)
“Wait? You saw Merlin?” I asked Bob.
“Nah,” Bob said, “but I cast Alec Guinness. Looks good, right?”
I sighed. “Could you get to the point, please?”
“Oh, come on,” Bob said. “I wrote in this romance triangle subplot and cast Jenna Jameson and Carrie Fisher. There’s a love scene you’re gonna really—”
“Bob!”
“Okay, okay. Fine. Sheesh.”
The movie shifted into fast motion. The grey-clad figure became a blur. It walked about waving its arms, and directed oceans of energy here and there, settling them all in and around the substance of the island itself.
“Wait. Did Demonreach tell you how he did that?”
“No,” Bob said, annoyed. “It’s called artistic license, Harry.”
“Okay, I get it. Merlin built the island. However he did it. Get to the part with the problem.”
Bob sighed.
Merlin walked into the woods in comically fast motion and vanished. Then time passed. The sun streaked by hundreds and then thousands of times, the shadows of the island bowing and twisting, the trees rising, growing, growing old, and dying. At the bottom of the screen, words appeared that read, A LOT OF TIME PASSES.
“Thank you for dumbing that down for me,” I said.
“De nada.”
Then the camera slowed. Again, Merlin appeared. Again, oceans of power rose up and settled into the island. Then Merlin vanished, and more years passed. Maybe a minute later, he appeared again—looking exactly the same, I might add—and repeated the cycle.
“Hold on,” I said. “He did it again? Twice?”
“Ah,” Bob said, as a fourth cycle began on the screen. “Sort of. See, Harry, this is one of those things that you’re going to have trouble grabbing onto.”
“Go slow and try me.”
“Merlin didn’t build the prison five times,” Bob said. “He built it once. In five different times. All at the same time.”
I felt my brows knit. “Uh. He was in the same place, doing the same thing, in five different times at once?”
“Exactly.”
“That does not make any sense,” I said.
“Look, a mortal jail is built in three dimensions, right? Merlin built this one in four, and probably in several more, though you can’t really tell whether or not he built it in a given dimension until you go there and measure it, and the act of measuring it will change it, but the point is: This is really advanced stuff.”

Cold Days Chapter 17, Page 166-168