Marked men are the remnants of the New California Republic and Caesar's Legion soldiers stationed at the Divide. The Courier unintentionally brought a package to the Divide from Navarro containing a mysterious ICBM detonator, causing the nuclear warheads that were still underground in their silos since the Great War to detonate. The result of the warheads exploding, on top of Big M.T.'s man-made disasters such as the earthquakes and violent storms present there that were an experiment gone awry, soon tore apart the entire area and all those residing within. The storms in particular had a profound effect on the NCR and Legion remnants; their skins were flayed off, leaving their skeletal muscle tissue exposed. At the same time, the radiation had caused a unique "ghoulification" process that had kept them alive, making them suffer from their heavy wounds. In dialogue between the Courier and Ulysses, it can be confirmed that such injuries and radiation would have killed ordinary ghouls. This makes the marked men unique in that it is that level of radiation that is keeping them alive.
Despite belonging to completely different factions with opposing ideologies when they were humans, the pain united them both into a single group. Their hate for the NCR and the Legion was turned against the Divide and its intruders. Some former legionaries of Caesar's Legion crafted crude versions of Legate equipment closely resembling Lanius's gear, using the metal from the Divide. Others shaped crude blades as weapons, in hopes of holding on to their identity that the Divide had snatched from them. Former NCR soldiers held on to their guns, raining fire on their victims from a distance.
Marked men are hardened ghouls with a fiery red color, their skins peeled off by the violent storms of the Divide. Because of this, their skeletal muscles are exposed, giving them their red appearance. They are considered ghouls, in that the intense radiation of their surroundings is keeping them alive. To protect their new skin (or more likely, themselves), they continue to wear tattered clothing from their respective factions. They maintain their clothing from scavenged pieces of scrap, such as fallen road signs, license plates, and other bits of metal. In order to feed themselves, marked men have resorted to cannibalism, evident in the hunks of human meat and flesh found nearby their encampments.
Marked men level with the player, have a high amount of health for humanoid opponents, and are moderately protected by the tattered armor they wear (albeit far less than properly maintained regular versions of their armor). They are overall on par with the White Legs tribals of Zion Canyon, and the lobotomites of the Big MT, in terms of health and armor.
They carry a variety of powerful melee and ranged weapons, and at higher levels (above level 40) have the unique attribute of being the only faction that almost universally uses non-standard ammunition for their weapons (i.e. slugs and armor-piercing rounds), increasing their combat proficiency against well-protected targets. Additionally, many marked men make simultaneous use of both firearms and grenades. At times, they can be found using Stealth Boys, making them harder to detect.
When near sources of radiation, they regenerate health rapidly, even more so than ordinary ghouls. However, they tend to be few in numbers, and it is rare to find more than a dozen in any particular location, unless the player is a high level.
Irradiated heavy troopers are the ghoulified remnants of NCR heavy troopers stationed at the Long 15 outpost. After the Courier launched the nuclear missiles from the Divide at the outpost, the radiation left behind turned them into ghouls, leaving them hostile to all others. They look like regular NCR heavy troopers with the added ability of regenerating health while irradiated.
The marked men that appear during the final battle will explode after being killed, with an explosive effect identical to that of a plasma grenade. This was done to prevent lag from too many corpses being in one area. It is still possible for it to lag by killing multiple marked men at close range with weapons like the shoulder mounted machine gun or frag grenades.
The Marked men are the some of the few enemies in game that will use special ammunition against the Courier.
In a possible conversation with Ulysses, the player can comment that many of the marked men appear to be NCR special forces (more specifically, rangers or other special combat teams).
Strangely, former NCR marked men seem to have lost their dogtags.
An additional, unused variant of the irradiated heavy trooper exists within the resources of Lonesome Road. Also called "irradiated heavy trooper", this version differs primarily in that it uses the default SPECIAL statistics for NCR troopers - 5 for all statistics. It also is equipped with a Minigun loaded with 5mm AP rounds, rather than the random weapon selection found in the standard version. Internally this version has the Editor ID NVDLC04NukeNCRColonel, indicating that this may have been an early stand-in for Colonel Royez.