The Warren is a location in Neithernor. When Deirdre first traveled to Neithernor, this is where she arrived.
According to Deirdre Green, the Warren was like one of those stories where animals had houses like people but made from things in the woods. The walls were soil but packed tight and rubbed smooth. The floor was made of flats of stone and nearly covered with overlapping mismatched carpets. There was a hearth charred by old fires. A wind down the chimney had blown cold ash across the carpets. The room was subterranean, but warm. There were ornate dressers and worn chests. The door to leave the warren was barred and broken, dirt and pebbles were scattered underneath it like a landslide had slammed it shut. There was only one window. Its frame was a tangle of knotted roots that grew out of the top of the wall and wreathed the opening, like someone had asked the tree for a window frame. It was too small to climb through. There was a narrow bed, which was unmade, and a small glass and metal lantern on the table beside it. There were paintings hanging on the walls. Landscapes. Seascapes. Shorelines and wooded paths. There was a painting of a chain of low islands on an ocean and in their center was what looked like a dozen shipwrecks smashed together. Crow’s nests on masts and bows and sails made a sort of looming castle in the middle of the sea.
Deirdre first encountered the warren in "My Mother’s Daughter," where Deirdre knocked into Neithernor for the first time. Knocking on the brownstone's door had led her to a broom closet in a warren, which she realized was thick with magiq. She fell asleep in the warren before knocking back into the mundane world, where she found a much shorter amount of time had passed. Deirdre learned that she had gained the power to summon a knocking door at will, allowing her access to Neithernor. The trip to the warren had unlocked a new journal entry, in which Sullivan described his pursuit of Neithernor and his life fleeing the Storm. He eventually found his way to Central Park, where he found the Storm couldn't follow him. A door to Neithernor existed there, as well as a the dying Council of the 18 Gates. Together, the two grew stronger and devised a plan to open the Book of Briars and stop the Storm once and for all. Sullivan had place his hopes in Deirdre, saying he knew that she would find her way to magiq because she was her mother's daughter. He told her to explore Neithernor and to take his Walking Stick with her, which he had placed near the warren's bed. After reading the entry, Deirdre and Cole had their first fight, because Cole didn't want to stay in the warren after it reminded him of a traumatic time from his childhood. Deirdre and Cole parted ways, with Deirdre continuing to explore Neithernor.
The warren acted as a secure place for Deirdre to return to. In "The Three Manners of Magiq," Deirdre noted she spent two nights there, spending the whole day watching the sky from the window. She attempted to open the door to the warren, but found that it was stuck.
In "Mr. Wideawake", Deirdre received a note that told her to meet them in the warren. Thinking it was Cole, she knocked in, but was surprised by a human-sized sooty tern bird named Mr. Wideawake. Mr. Wideawake told Deirdre that he was the current Collector for The Monarchs and that, if the Council and the mountaineers were wiped out, him and his associates were the last thing standing between the Silver and complete control over magiq. He explained to Deirdre how the Book of The Wild had been altered to erase magiq in what became known as the Book of Kings. Mr. Wideawake told Deirdre that the reason he was there was that the Monarchs wanted to preserve The Little Red House at any cost and that they wanted to stop the Mountaineers, who were planning to exchange it with the Storm for the Book of Briars, so that they could study it and understand how it fit in to the changes made in the Book of Kings. The message was actually a warning to Deirdre that the Monarchs were coming for the book, since they knew where to find it and how to access it using her blog post.