The Nail is a paranatural object located in the Crossroads area of the Foundation in the Oldest House. It is a towering column of the same kind of black and gold stone found in the Astral Plane that extends deep into the Lower Crossroads. The Nail is described by Emily Pope as either a fragment of the Astral Plane or a container for it, or possibly both simultaneously.
The Nail appears to work as a tether, connecting The Board and its realm of the Astral Plane to the Oldest House. In addition to allowing passage between the real world and the Astral Plane, it possibly allows The Board to exert control over the Oldest House's internal architecture. Emily Pope observed that The Nail continuously emits low-level radiation throughout the Foundation, which prevents any organic life from growing there and causes symptoms of neural disruption, such as anxiety and agitation, in humans who experience prolonged exposure to it.
The Lesser Nail
A small-scale copy of The Nail is located in the Basement level of the Warehouse structure in the Foundation. As the main Nail tethers The Board to the Oldest House, this lesser Nail appears to do the same for Former.
The Federal Bureau of Control discovered The Nail in 1964 during its early expeditions into the Oldest House. Director Broderick Northmoor also found the Service Weapon on a pedestal that appeared near The Nail, beginning the FBC's association with The Board. After Dr. Theodore Ash, Jr., then Head of Research, learned how to harness Control Points to stabilize the upper layers of the Oldest House, Northmoor ordered the Foundation evacuated and sealed off, and the existence of both the Foundation and The Nail became a closely-guarded secret until their existence was brought to light by Director Jesse Faden in 2019.
During the 1964 expedition, Theodore Ash, Jr. found the lesser Nail and communed with Former, and became convinced The Board was a parasitic entity that was manipulating the FBC for its own incomprehensible purposes. Ash refused to evacuate with other FBC staff and stayed in the Foundation, studying the Nails until the "Id" became too violent. When he finally returned to the Oldest House's upper levels, he resolved to try and correct the FBC's course away from The Board, but did not succeed.
During the Hiss invasion in October 2019, Federal Bureau of Control Head of Operations Helen Marshall attempted to destroy The Nail to sever the Oldest House from the Astral Plane and protect the FBC from The Board's machinations. She managed to damage The Nail with explosives, but this caused the Astral Plane to begin leaking into and subsuming the real world. In retaliation, The Board sent an Astral Spike against Marshall, which damaged her Hedron Resonance Amplifier and exposed her to possession by The Hiss. Jesse Faden later entered the Foundation to repair The Nail by restoring four damaged keys in the Astral Plane in a ritual, and then cleansed The Hiss from The Nail. Faden learned of Marshall's fate and wondered if The Board had deliberately let The Hiss infect The Nail, but could not fathom why, and decided to quietly lead the FBC in a direction free of The Board's influence.