The Codex is an alien and a contestant in the seventh Presidian.
Since the first Presidian began, the Codex's species were considered intelligent enough by the presiding race to be allowed to participate. When it came time for the Seventh Presidian, the one that would simply be called the Codex (despite that being its species' name) was chosen, and teleported to the labyrinth on Earth with its guide.
The Codex was teleported into the New York Public Library to partake in the Presidian and would later find Paul Hawkins and Balthazar holding up in a storage facility. The Codex attacks and Hawkins sets fire to the room, hoping to deter the Codex from, which fails. Soon Hawkins realised that the room was filled with several flammable chemicals, and Balthazar, already struggling with the Codex, ordered Hawkins to flee.
After the room exploded, Hawkins saw the Codex stumble out of the room. Though he initially panicked that the explosion had barely injured it, the Codex soon collapsed, dead.
The Codex are black-colored aliens, with a floating triangular head, and a body and tail of trailing spheres. It moves by silently hovering through the air, and attempts to strangle its prey.