A Roman centurion who had died in battle is resurrected from the dead by Zeus. The centurion is ordered by Zeus to save his daughter Athena from a Demon God called Neff in the Underworld. To become able to withstand the perils, the warrior gets the ability to absorb spirit balls which transform him into an Altered Beast, a part animal, part human creature of formidable force. Between each level are small animations giving the player glimpses of Athena's peril. He first travels through the graveyard fighting past zombies and winged demons. The centurion becomes a werewolf in order to fight Neff in the form of Aggar. The centurion defeats him, but Neff escapes.

The centurion travels through the Underworld where he fights through giant leeches, Chinese dragon-headed rattlesnakes, and ferocious Chicken Stingers (resembling the ones from Golden Axe). The centurion turns into a humanoid thunder dragon in order to fight Neff in the form of Octeyes. The centurion defeats him, but Neff escapes.

The centurion travels through the Cavern of Souls where he fights through Cave Needles (which resemble giant ants), turtle/snail hybrids, and zombies known as Grave Masters. The centurion turns into a humanoid bear in order to fight Neff in the form of Mouldy Snail. The centurion defeats him, but Neff escapes.

The centurion arrives at Neff's palace where he fights through zombies, Chicken Stingers, and Hammer Demons. The centurion turns into a humanoid tiger in order to fight Neff in the form of Crocodile Worm. The centurion defeats him, but Neff escapes.

After the centurion arrives in Dis, he fights through winged demons, Goat Men, Boar Men, saw fishes, and purple Unicorn Men to get to Neff. When the centurion confronts Neff, the centurion becomes a golden werewolf while Neff transforms into a Rhinoceros-Man. After he is defeated, a blue bird comes out of the ground and transforms back to Athena. Afterwards she is shown holding arms with her lycanthropic hero, thanking him for rescuing her.

In the original arcade, the end credits are interspersed with images of actors in costumes for the different characters and monsters of the game, implying the whole game was a film production.

• Graveyard - The first level. Consists of zombies of the sort. The player turns into a Werewolf in this level.
• Underworld - The second level. Consists of giant leeches, Chinese Dragon-headed rattlesnakes, and ferocious chicken stingers (resembling the ones from Golden Axe). The player turns into a Thunder Dragon in this level.
• Cavern of Souls - Consists of giant ants called Cave Needles, turtle/snail hybrids, and zombies known as Grave Masters. The player turns into the Bear Man in this level.
• Neff's Palace - Consists of zombies and Hammer Demons. The player turns into the Tiger Man in this level.
• The City of Dis - Consists of Goat Men, Boar Men, saw fishes, and purple Unicorn Men. The player turns into a Golden Werewolf in this level.

Bosses

Neff will turn himself into 5 different monsters on each level. Among the 5 monsters are:

• Aggar - An ogre-like monster that is primarily made up of earth and rotting corpses. It has endless heads, which it can rip off and can throw at the player.
• Octeyes - A plant-like entity that attack the player with huge amounts of poisonous spores in the shape of eyeballs.
• Mouldy Snail - Part snail, part salamander, It has sharp bone-like protrusions from its shell, and can attack with streams of green fireballs.
• Crocodile Worm - A fiery-bellied crocodile-headed monster that can breathe fire and summon small fire dragons to come after the player.
• Rhinoceros-Man - Neff himself takes on the form of an armored rhinoceros-like creature that can charge across the screen at high speed.

Altered Beast was produced as a standard upright only with custom artwork on the cabinet. In most versions, the game's controls consist of an eight-way directional and three buttons, one each for "punch", "kick" and "jump". The game has single player and cooperative two-player modes. One of the more well-known features of Altered Beast is the use of voice samples, such as Zeus saying "Rise from your grave" and Neff's "Welcome to your doom!".

Altered Beast was ported to several platforms after its original release in 1988. It was released for Sega Master System, PC Engine, PC Engine-CD, Famicom, Atari ST, ZX Spectrum, Commodore 64, Amstrad CPC, Commodore Amiga and DOS. The Sega Mega Drive/Genesis version is noteworthy for being the original pack-in game for that system in North America, Europe, and Brazil, before being replaced by Sonic the Hedgehog. A hand-held version of the game, made by Tiger Electronics was released in 1988.

The Mega Drive version is included in the compilations Sega Smash Pack, Sega Genesis Collection and Sonic's Ultimate Genesis Collection, with the latter two also including the arcade version as an unlockable game. The Wii's Virtual Console service offers emulated versions of both the arcade and the Mega Drive port, while the Xbox 360's Xbox Live Arcade and the PlayStation 3's PlayStation Network have a re-worked arcade version with HD support, online leaderboards and network play. Sega released an official iOS port of the Mega Drive version in late 2010, played on the iPhone and iPad.

Certain differences are seen between the several versions of the game. Some of them, like the Master System one, were only single player and had only four levels. Others provided different beasts to mutate into, such as a humanoid lion or a shark form seen in the NES version.

In its initial arcade release, Altered Beast was a well-received game. Its conversion to the Sega Mega Drive was considered inferior to the arcade in terms of sprite quality. However, the Sega Mega Drive version was actually more advanced than the arcade version in one regard - its utilization of parallax scrolling. Mega placed the game at #1 in their list of the 10 Worst Mega Drive Games of All Time. Its re-release for the Wii's Virtual Console was given a lukewarm reception by GameSpot and IGN, describing the game as merely decent with some nostalgic value. The Xbox Live Arcade rerelease was even described by IGN as "relic of the arcade heyday that just doesn't hold up today".

Project Altered Beast

A PlayStation 2 title was released by Sega in 2005, known as Jūōki: Project Altered Beast in Japan and simply Altered Beast in Europe; the game was not released in North America. In Japan, it was rated 18+ for its intensity; however, the PEGI system rated it 12+, while none of the extreme gore animations were censored.

Rather than serving as a sequel to the original game, the newer title features a more modern setting that is unconnected to the original game. The story follows a man called Luke Custer who is a "Genome-Cyborg", which in the game is a human that has had his DNA and other genetic make-up altered so that he can (after acquiring the chip containing the right genes for each creature) transform into a mythical beast. The main creatures he transforms into are a Werewolf, a Merman, a Garuda, a Wendigo, a Minotaur, and a Dragon. Its storyline differs from the original Altered Beast games where during the game, you gain a serum that allows you to change at will. There are a number of difficult boss challenges and puzzles within the game, plus some extra characters to transform into including a Weretiger, a Grizzly Bear, and the alien-cloned U.W.H. (Unidentified Weightless Human) for those who finish the game.

Altered Beast: Guardian of the Realms

Altered Beast: Guardian of the Realms, developed by the now defunct 3d6 Games and published by THQ, is a sequel for Game Boy Advance in the style of the original arcade game. It adds new features like power-ups, new beast forms and destructible environments.

In this game, you turn into a Werewolf (called Canis), a Snake Man/Nagi (or Naga), a Dragon (or Draco), a Tiger Man (or Smilodon), a Spiked Turtle (or Terapis/Terrapin), a Shark Man (or Carcharodon), a Rhinoceros Man (or Cerathos), an Eagle (or Avion), a Scorpion (or Scorpios), and a Chimera.

A pink enemy creature, known as a cockatrice, was later depicted as a mountable character in the game Golden Axe.

Altered Beast was also the subject of a skit in the second season of Mega64.

On Friday Night SmackDown, Matt Striker often called Batista "The Altered Beast" in addition to calling him "The Animal", in reference to Batista turning heel and showing a drastic change in his character compared to his face character.

Several musical groups have paid tribute to Altered Beast in some form. MegaDriver released an album based on the game's music, Metal Beast, in 2004, with also a patch to include said soundtrack to the arcade version on MAME. Matthew Sweet named an album of his Altered Beast after the game in 1993.

Robby Roadsteamer's 2008 album "LRP" features a song named "Altered Beast", as does We Are Scientists' second album, Brain Thrust Mastery, as does the White Fields & Open Devices album by Vessels; the game's name also serves as the name of an LP released by Additionally, Hello, Sailor released a song titled "Hand Vs. Battle Axe" that includes the "POWER UP!" sample. The Mr. Bungle song "Egg" samples the "Rise from your grave" line from the beginning of the first level. Alternative metal band Breaking Benjamin created a flash game entitled "Altered Benjamin" in which you play as vocalist Benjamin Burnley and is an obvious parody of Altered Beast. It is currently available for play on the band's website. Sydney based instrumental prog trio Altered Beast are named after the game and also reference various bits of in game music in the song "Super 7 Medley". The phrase "Welcome to your doom" is sampled by dubstep artist Reso in his track "Beasts in the Basement". On rapper Ludacris' 2nd studio Album "Word of Mouf", the titular track has rapper 4-ize make an homage to the classic game during his freestyle verse when he says "I rock Shaq's clothes when I Alter Beast, Power up, get big it's disturbin the peace". The song Genesis 16:12 by The Left Rights references Altered Beast, specifically "Rise from your grave."

Neff's Rhinoceros-Man form makes a cameo in the Walt Disney Pictures film Wreck-It Ralph. He is among the villains seen attending the villain support group Bad-Anon and a Sardi's style caricature of both Neff and Centurion appear on the walls of the Tapper's Bar.

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