The Hidden uses Arena rules, but pits a lone BLU Spy (as the titular "Hidden") against everyone else on the server.
The lone Spy possesses some special abilities, such as invulnerability to afterburn and bleeding, increased health, the ability to leap, cling to walls, and speed, as well as a longer Cloak duration and ability to "Scare!" (causing the enemies to be stunned, akin to the Boo! taunt from the Horseless Headless Horsemann) enemies which slowly recharges after use.
The "Hidden" is cloaked at all times, apart from when attacking or leaping. The Cloak slowly drains, but is replenished if the "Hidden" makes a kill. The "Hidden's" health is also slightly replenished upon successfully killing an enemy. If the "Hidden's" Cloak meter drains completely the "Hidden" will die automatically.
Players on the opposing team of the Spy have a green outline similar to that used by the coaching system, and akin to the one used in the original mod.
Players killed by the Hidden suffer a distinctive death by disintegration: they turn black, hover in mid-air while performing a specific disintegration animation surrounded by team-colored particles, and then disappear into thin air. A message is also displayed on some servers to every player saying "The Hidden killed <player> and ate his body".