Locke & Key

This plot is presented in chronological order.
In 1775 during the American Revolution, a group of Rebels, hiding in a sea cave in Massachusetts Bay beneath what will one day be Keyhouse in the town of Lovecraft, discover a portal to another dimension, the plains of Leng. It is filled with demons who can mesmerize anyone who sees them and possess them through touch; two men are possessed over two days, and they kill several rebels. However, when the demons attempt to enter the real world without possessing someone, they are transformed into "whispering" iron, which young smith Benjamin Locke forges into a variety of magical keys. The first key he makes is the Omega Key (which has an Omega character on the head) that seals the entrance to the dimension. The magic of the Keyhouse gradually evolves over the years, including a spell that causes occupants to forget about the keys and the magic of the house when they pass their 18th birthday. In 1988, a group of teenagers, having used the keys extensively in their high school years to their great delight, decide to open the black door with the Omega Key, hoping to trick a demon into entering the real world in order to provide more metal with which to make more keys. Rendell Locke's younger brother Duncan follows the group into the cave, but he is mesmerized by demons in the open door, and attempts to walk through it. He is stopped by Lucas "Dodge" Caravaggio, who accidentally puts his hand through the doorway and is possessed by a demonic being. He plots to kill his friends and enslave the others; Dodge kills several and causes another to lose her memory. He also extracts a part of his spirit to be hidden in a jar in the home of Ellen "Ellie" Whedon, one of the surviving friends, as a contingency plan. Dodge is finally killed by Rendell before he can force more people to be possessed. Rendell discovers a small piece of "whispering" iron from the sea cave and disguises it as a fishing lure.
Around 2010, the fragment of Dodge's spirit partially possesses Ellie, who then conjures the rest of Dodge's spirit in the physical world at the magical well at Keyhouse, but Dodge is trapped within the well house. In 2011, Dodge's spirit reaches out to a young, abused prodigy, Sam Lesser, and convinces him to force Rendell to give him the Anywhere Key (which will allow Dodge to escape the wellhouse) and the Omega Key. When Rendell refuses, Sam murders him, but Sam is then incapacitated by Tyler, Rendell's oldest son, and captured by the police. The Locke family, children Tyler, Kinsey, Bode, and mother Nina, move across the country to Keyhouse, reuniting with Duncan, and begin discovering the house's secrets. Sam escapes prison with Dodge's help and follows the Lockes to Massachusetts. Sam attacks the family again at Keyhouse, but Dodge also tricks Bode into bringing him the Anywhere Key in case Sam fails again. Dodge escapes from the wellhouse with Bode's help, kills Sam, and returns to Lovecraft in the same body as he had thirty years before.
Dodge intimidates his way into living in Ellie's home, who is now a teacher at the same high school she originally attended with Dodge, and where Tyler and Kinsey are enrolled; Dodge also re-enters the school under the guise of a new student and Ellie's cousin. Over the next year, Dodge secretly tries to recover the various keys – particularly the Omega Key – from the children, collecting many, though hindered by Tyler and Kinsey. Dodge is eventually discovered but manages to switch bodies and possesses Bode before they can kill him. Now free to explore the house as Bode, Dodge finally finds the Omega Key and plans his takeover during the after-prom party in the sea cave. Demons possess several students, and many others are killed. Dodge is ultimately undone by Tyler, having re-discovered the "whispering" iron from his father; he forges an "Alpha Key" with Duncan's help, which can exorcise a demon from the possessed soul, though it is fatal to the physical host. Dodge's spirit, in Bode's body, is forced back into the well killing the spirit again, though Bode's empty body is cremated before Bode's soul has a chance to return into it.
In the epilogue, Tyler conjures Dodge's spirit again and uses the Alpha Key to finally exorcise Dodge from Lucas' soul, allowing Lucas' unpossessed soul to finally rest in death. Tyler uses the well's magic to speak with his father one last time. Finally, Tyler restores Bode's spirit to his physical form using the Animal Key.