Ghouls are former humans who have become horribly irradiated due to the radioactive fallout covering the wasteland from the Great War. Many ghouls are pre-War humans who survived the initial nuclear bombardment during the Great War. Becoming permanently irradiated, they were horribly disfigured and their lifespans were increased virtually indefinitely. Eventually, the constant 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. Understandably, 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 to quickly dispatch them and handle any uninjured ghouls. Rarely, a feral ghoul will still attack after its head is decapitated. It is unknown if this is a bug.
A notable difference between the Massachusetts feral ghouls and feral ghouls 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 rise. 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. 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, they often fall down 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.
They very rarely travel alone, and it is quite unusual to encounter a single ghoul. This behavior has lead 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.
Stalkers often pretend to be dead or hide amid rubble until an enemy is detected, where they then attempt to ambush their prey. 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.
Reavers usually have pieces of post-war metal armor attached to them as in Fallout 3 and New Vegas, possibly implying these are rather recent victims of ghoulification. As in New Vegas, 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.
Withered feral ghouls are slightly skinnier, with a 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.
Gangrenous feral ghouls are more rare and considerably stronger than the previous variants. They can be hard to defeat even for some mid/high level player 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 also exists within the O'neill family factory in the Glowing Sea.
Rotting feral ghouls have a bloated appearance, indicating that their bodies have began 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.
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 players, 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. It gives off radiation the closer one is, even more so when hit by it. This is the strongest type of Feral 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 ferals.
Curiously, feral ghouls are not hostile towards cats. This might be due to the game's data which does not allow the Sole Survivor to interact with cats and, as a consequence, also does not allow other characters to do so as well.
Feral ghouls' limbs are severed when broken compared to other creatures. Against a crowd, they can be safely incapacitated with less effort by severing their legs first, and only disposing them when it's safe to do so.
Feral ghouls can continue to survive, even if (the top part of) their head has been blown off (however this is vary rare).
Feral ghouls will randomly become friendly if you have the Ghoulish perk.