Ghouls are former humans who have become horribly irradiated by the radioactive fallout covering the wasteland from the Great War. Many ghouls are pre-War humans who survived the initial nuclear bombardment. Becoming permanently irradiated, they were disfigured and their lifespans were increased virtually indefinitely. In most cases, exposure to radiation in the post-War Commonwealth wastes degenerated their brains, causing them to lose their higher cognitive functions, turning them into mindless, radiation-resistant cannibals.
The degeneration into a feral state (referred to as ferocious post-necrotic dystrophy) is not fully understood. It is known that it is a result of a degeneration of the brain (which is not affected by the regenerative mutation of the spinal cord) and emerges following the atrophy of higher brain functions, accompanied by an increased level of aggression and appetite. When the loss of capacity for thought is complete, a ghoul is considered feral. The factors that may cause a ghoul to enter this state are unclear, but anti-social or isolated ghouls are more susceptible to it. Exposure to intense radiation may also be a factor; feral ghouls have no body heat and emit lethal levels of radiation, indicating that feral ghouls are for all intents and purposes dead, their functions sustained only by their high irradiation. This allows them to survive for centuries, relying only on the energy provided by radiation, at the expense of a horrifying, emaciated appearance. The process is irreversible.
It is now possible to dismember ghouls, however, they will continue to chase and bite the Sole Survivor even if they lose both of their arms. When losing a leg, however, they will become docile and will no longer try to attack, allowing the player character to quickly dispatch them and handle any uninjured ghouls. Rarely, a feral ghoul will still attack after its head is decapitated. Feral ghouls being exposed to high levels of radiation such as those present in the Glowing Sea will rapidly regenerate HP.
A notable difference between the feral ghouls found in the Commonwealth and those seen throughout New California, the Mojave Wasteland, and the Capital Wasteland, is that they have glowing yellow eyes. Sometimes feral ghouls will lie on the ground, either to rest or play dead. Once the player character gets close enough or they are otherwise alerted, they get up and attack. These ghouls can sometimes be targeted in V.A.T.S. and if spotted, attacked before they arise. They also show up as living actors on the HUD if the Sole Survivor is wearing power armor with a targeting HUD-equipped helmet or aims at them with any kind of recon scope.
Occasionally, they will start to run faster at the player moving erratically to dodge incoming attacks, suggesting some form of intelligence, and will eventually lunge at their attacker once they're close enough. A gurgling roar is heard when a ghoul charges at the Sole Survivor. This attack can sometimes be difficult to dodge without sprinting, but it can, however, be blocked or interrupted with a melee strike. Additionally, when feral ghouls lunge at the player character, they often fall beside or behind their target, giving the player a brief chance to strike while the ghoul is getting back to its feet. In some locations, particularly indoors, ghouls will also emerge from windows, ductwork, etc., to attack when a victim comes close. Ghouls hiding in this sort of ambush cannot be detected even by V.A.T.S. before they begin emerging - however, while they are emerging they are helpless while climbing into the room and can be freely attacked if one is quick. Often they will not detect the player character until they finish emerging.
They very rarely travel alone, and it is quite unusual to encounter a single ghoul. This behavior has led to groups of ferals often being called "packs" or "hordes" by wastelanders. Another notable difference from previous versions of ferals is that they inflict radiation damage as well as physical damage.
Typical feral ghoul, exhibiting zombie-like behavior. These are more than likely the first type of creatures the Sole Survivor will encounter as they can be found all over the Commonwealth. Aboard the Yangtze, unique feral ghouls called crewmen can be found.
Stalkers are a variant of feral ghouls that can easily be distinguished by their bloody mouths and paler skin. They often pretend to be dead or hide amid rubble until an enemy is detected, where they then attempt to quietly ambush their prey without revealing themselves, usually with success. There are generally at least a few stalkers mixed into larger groups of weaker ferals. As they wear the same tattered clothing as roamers and normal ferals, they can be difficult to spot until they're damaged. They will also sometimes serve as a "boss" in locations populated by ferals. If the player character is of a high enough level, the boss may instead be a glowing one, which the stalker will still accompany.
Reavers usually have pieces of post-War metal armor attached to them. They lack a projectile attack and are generally the ones in a pack to reach better-equipped targets thanks to their respectable amount of HP. They are much easier to deal with than past versions. Unlike the ones in Fallout 3 and New Vegas, their corpses do not emit green fumes or have boil covered skin.
Withered feral ghouls are slightly skinnier, with a yellowish leathery-looking skin, and wear no clothes. These ghouls are likely fairly old compared to others, as their clothing has rotted away and they appear to be severely sun-damaged. A withered feral ghoul can be found inside the power armor storage in the national guard armory. Their eyes and mouth also appear severely damaged and shriveled.
Gangrenous feral ghouls are considerably rarer and stronger than the previous variants, with torn rags like common variants, and yellow skin. They can be hard to defeat even for some mid/high level player characters due to their aggressive attacks and high health. A gangrenous feral ghoul can be found inside Croup Manor's basement guarding a chest with some expensive loot. One can also be found at O'Neill Family Manufacturing in the Glowing Sea.
Rotting feral ghouls have a bloated appearance, indicating that their bodies have begun decomposing, also suggested by the name. Although their offensive capabilities are on par with gangrenous feral ghouls, they have almost twice the amount of health.
The highest tier of all non-glowing feral ghouls, this variant is covered by a thick layer of soot and is much tougher to kill than a normal feral. These level with the player character, making them immune to the Wasteland Whisperer perk.
A highly irradiated version of a feral ghoul, they glow with green light emitting from within, and just being near them causes radiation damage. They can perform an area of effect radiation attack indicated by the raising of their arms which releases radiation around them. This radiation also heals nearby feral ghouls, and glowing ones can even sometimes revive recently killed allies. They are potentially more dangerous than past iterations, especially against low-level player characters, due in part to their radiation-fueled healing, although their passive radiation radius is smaller and thus they are easier to dispatch without taking any radiation.
A higher level version of glowing one, covered in glowing green blisters of radiation. Its radiation aura is almost twice as strong, but it is no longer capable of performing its area of effect radiation attack. This is the strongest type of feral ghoul in the game, damage-wise.
A higher level version of glowing ones. Its body is bloated and swollen with radioactive material. It gives off more radiation than its counterparts and the melee attack is a little weaker, compared to a putrid glowing one. However, it does have the highest health of all feral ghouls.
A variant only found on the Island and mutated by the island's radioactive fog. Fog ghouls are much paler than ghouls found in the Commonwealth, with black burns on certain parts of their skin and fishing nets draped over their chest. Their eye sockets are quite large, with the eyes sunken in, and their teeth are permanently gritted, exposing the gums, which seem inflamed, and the teeth sunk in as well. Black soot is also smeared across their faces.
↑The Lone Wanderer: "I need more details... What's the breach all about?" M.A.R.Go.T.: "Internal visual sensing equipment is detecting numerous unidentified persons in the southeast tunnels area. The individuals have damaged metro equipment and refused dialogue with our security units. In accordance with U.S. Security Statute A567/B, the use of deadly force has been authorized." The Lone Wanderer: "Can you tell me more about these unidentified persons?" M.A.R.Go.T.: "My apologies. My sensing equipment in that area must be damaged. My equipment indicates the persons possess no internal body heat and are emitting lethal levels of radiation." (M.A.R.Go.T.'s dialogue)