The Monument (formerly known as The Thorn) is a cannibal-controlled section of the New Vegas Sewers, located underneath Westside.
Background
Prior to the Fall, the Thorn was an arena for pit-fights where dangerous specimens of Mojave wildlife were pitted against one another or against human challengers for the enjoyment and enrichment of spectators. While the specific fate of the Thorn and it's operators is unknown, the Vault 21 lab policy of discarding failed experiments in the New Vegas sewers in conjunction with the incipient civil war under the New Vegas Strip likely forced the locations disestablishment well before the events of Fallout: Dust, (though the management may have simply focused their bloodlust elsewhere). In any case, when the sewers were taken over by opportunistic cannibals, they established their main base of operations in the Thorn's former arena, constructing the eponymous monument, a macabre obelisk of flesh and bone with which they would lure unwary travelers to their doom.
Overview
The monument dominates the northwest section of the New Vegas Sewers. Being the centerpiece of the Cannibal's ruse, it is heavily occupied by well-armed cannibals, many of whom are clad in metal armor or use high-power factory-machined firearms. The monument itself is located in the Arena area, with containers at its base containing dozens of copies of the cannibals trap key, the actual sewer master key and various miscellaneous items undoubtedly procured from the cannibals previous victims. The area is ringed by a sewer spillway which contains cages containing human cattle.
Notes
The Thorn's previous entrance south of Westside is no longer usable as a point for entering or exiting the New Vegas Sewers.
Insane survivors will encounter Shadowclaws in the cage area. These are immediately hostile and will cause a blackout upon their attack.
Trivia
As of DUST 2.0 Red Lucy's private quarters are accessible. Insane characters will encounter a single spirit inside.
Leaving and reentering the cage area will cause ants, cazadores and tunnelers to spawn alongside the human cattle there. These creatures cannot not hostile to either the human cattle or the player and can be killed at leisure for experience.
The presence of Tunnelers in the cage area is a consequence of all gecko spawns in the base game being changed to tunnelers for DUST.