Zombie or Mutant(as described in HotD: Overkill) are a variety of corpses resurrected by the mutagen created by Dr. Curien. These reanimated specimens vary in appearance and attacking methods. While new creations have spawned throughout the game, there are several familiar zombies that reappear in the sequels.
All specimens vary in appearance, depending on the time of death and resurrection, and forms of attacks. The standard ones walk with a lumbering slowness but are deadly in groups while some other types possess superhuman capacities such enhance strength and agility. Unlike zombies featured in other games, these zombies have shown minor intelligence, such as being able to utilize terrain and weapons. Some even feature to be wearing armor. Another departure from traditional zombies is that many of these zombies seem to be focused more on killing the victims, rather than eating them. However, there are cases where some zombies(evident in HotD III) where they can be seen feasting on some form of food source.
It is revealed that, 40 years ago, a group of military scientist were developing a formula known as Formula X during the Cold War for the sole purpose to alter a person's genetics transforming an ordinary soldier into a physically enhanced super soldier. Unfortunately, the project failed and was still considered experimental at the time. Remaining samples of the formula are said to be hidden in an abandoned military bunker and can be assumed that those samples could possibly be the base mutagen formula that created the resurrected deceased. This is well portrayed by highly enhanced zombies that possess superhuman durability and retain whatever is left of their former human lives such as the ability to perform acrobatic stunts and utilizing objects as weapons.
The improved(speculated) mutation compound has then been used and tested by certain powerful figures creating countless monstrosities. Since the formula has secretly continued development by different said figures, the mutation has undergone major overhauls that catastrophically result into rewriting human biology. This is most evident by the Bosses in each end level of the game. Each extremely mutated abominations aren't only large in size and shape but also possess feats beyond human competencies. Apparently, humans aren't the only organisms to be affected. Anything living such as animals, insects and plants can succumb into the formula's destructive alteration.
The exact procedures of mutation are unknown but there are evidences that reveal how the dead came back to life:
- The zombies are specimens kept in test chambers. As said before, the formula is still experimental and would probably require it's victim to be in the state of necrosis. From there, there are a multitude of possibilities:
- They would be fused with other living things to deliver severe damage.
- They would be granted the familiarity of wielding weapons and perform stunts.
- Granted with abilities possibly described as supernatural.
- Newly awakened corpses are assumed to be infected by the mutagen in an airborne state and will be resurrected after death.
The main reason why these monsters are created is due to humanity's inevitability to fall into their own demise. Since humans are the cause of disasters, both man-made and naturally occurring, such as wars, famines, poverty and global warming, the zombies were formed to wipe out most of the human population. This purpose has been proven successful. In the events after HotD II, there are barely any humans inhabiting the world they use to live in. In HotD III, the aftermath of the outbreak has revealed that the earth in now an apocalyptic wasteland.
- Regular - Slow, weak and often coming in various shapes, the regular zombies are usually the most common enemy in the game. They are always unarmed, and are easily put down with one headshot in most of the games. They attack by biting, clawing, kicking and shoulder-barging the player.
- Muscle/Soldier - Bigger and tougher than Regulars, these zombies are noticably taller and more muscular than the others. They often appear shirtless, or in semi-military attire (In the second game some wear military berets, in the third some have camoflauge pants and a flat-top haircut.). They take significantly more firepower to take down, and usually attack by punching or bludgeoning the player.
- Fatmen - Like the name suggests, these zombies are encountered in every game. Like the muscle zombies they take a lot of firepower to bring down, and are often seen fighting alongside them. They often toss barrels at the player from a distance. Suprisingly, and humourously, some move and attack like undead ninjas - they sometimes sidestep towards the player like a boxer before delivering a punishing spinkick.
- Axeman - Appearing in every game, the axeman zombie comes armed with a pair of axes/hatchets. They use these weapons to shield their vulnerable heads, prompting the player to aim for the arms or the chest to expose the cranium. At a distance, they will lob both their axes in rapid succession at the player, which can be shot down. A unique type appears in the third game - it only has one large axe, and is unable to throw it.
- Kageo - Appearing in all games (except Overkill), these lean, rotting zombies often appear in dreadful dark areas such as sewers and abandoned laboratories. They are among the weakest in the game, taking only a few shots to dispatch. They have a consistent appearance throughout the games, a hunched-over swagger with medium-length balding hair and rotting skin. There is also a crawling half-bodied variant known as Kageo J.
- Ebitan - Appearing in all of the games (except Overkill), Ebitans usually prefer to stay in the water. Their attacks often involve leaping out of fountains and canals to attack the player. They take a lot of damage, and move as fast as Regular zombies even on land. In the third game, there appears to be a smaller variant, which shuffles toward the player before leaping at him/her.
- Knife Throwers/Leapers - Arguably the most annoying undead enemies, the Knife Throwers and Leapers are sometimes fused into one (irritating) type of foe, although in the second and third games they appear to be seperate. The Knife Thrower will usually move around at a rapid pace dodging the players attacks, advancing until it can take a slice at the player, all the while lobbing an infinite supply of sharp things. The Leaper, on the other hand, usually travels with a buddy, and likes to move even quicker, climbing along walls and hanging from the ceiling to reach the player quicker.
- Chainsaw Zombie - Probably the toughest enemy in all of the games, the Chainsaw Zombie runs at the player relentlessly to cleave them in two with his motorised death-machine. They have possibly the highest health of the zombies, making it all the harder to bring them down before they will deal damage.
- Murrer - An ugly, slimy worm-like creatures that crawls around in packs, the Murrer will slither around for a bit before jumping to bite the player. They are mostly just a nuisance unless theres a ton of them, which there usually is.
- Animal Zombies - Bats, owls, frogs, vultures, giant cockroaches, massive green Harvestmen spiders. They can become dangerous if not disposed of quickly.
- Sledgehammer - Unique to the first game, the sledgehammer zombies appear in two types. One wields a huge sledgehammer and the other one is using some sort of a gigantic mallet on a chain (this type almost always attacks from the distance).
- Broadsword - Unique to the second game, this particular zombie wields a massive broadsword, which he often uses to protect his face while advancing. It becomes vulnerable when (slowly) swinging the massive weapon around.
- Lizardmen - Unique to the second game, these peculiar lizardlike undead hang from pipes and whip the player with their "tails". They possess unusual resilience for their size, being able to get back up and continue attacking after being knocked down multiple times. Regardless of the settings, their blood is always dark, muddy green.
- Parasite - Unique to the second game, the parasite zombie is infected with a giant worm in its chest (which may be a reference to the Chestburster from the Aliens series). If shot anywhere other than the head, it will become temporarily invulnerable before ejecting the worm from its chest and dying. Its toughness is the same as a regular zombie - with good aim, it will prove to be no more dangerous than one.
- Commando - Unique to the third game, the Commando zombies are Rogan's men who turned undead after storming the building and getting killed in the process. Commando zombies attack with impossible speed, phasing from side to side and rolling around before slashing at the player. They are difficult to hit, but sometimes one good shot would be enough. A unique type - undead version of Yukio (a character who dies in the beginning of Chapter 0) is found in the last level of HOTD III, taking a a lot of damage and standing back up multiple times before succumbing. There is an alternative to Commandos in the fourth game, who are undead AMS agents.
- Crawler - Unique to the third game, the crawler hangs on to the ceiling and attacks by descending from above and clawing at the player. They are usually encountered alone, and at rare occassions in a pair or a trio.
- Flyer - Exclusive to the fourth game, this mutant zombie appears to be a cross between human and bat or some bird. They are not that hard to deal with, but due to their gliding in the air it may be difficult to hit them sometimes.
- Cybrid Zombies - Exclusive to the final levels of the second and fourth games, the Cybrid Zombies are upgraded, or scratch-built, corpses. They take more damage than other zombies, some of theme even have special abilities like being invisible and "teleporting" around.