The Six Arms Saga

"The Six Arms Saga" is a story arc featuring the popular Marvel Comics character Spider-Man, written by Stan Lee and drawn by Gil Kane. It spans the issues "The Amazing Spider-Man" #100–102 (1971) and features the first appearance and origin story of Morbius, the Living Vampire. The story arc is mostly remembered for Spider-Man growing four extra arms.
Plot.
Peter Parker has recently experienced a lot of misery in his life: his best friend Harry Osborn has become a junkie, his close friend Captain George Stacy died in his arms, and Gwen Stacy – Peter's girlfriend – wrongly blamed Spider-Man for her father's death. He has a fatal thought: "for Peter Parker to live, Spider-Man must die!" Peter has been working on a serum to terminate his spider powers ever since he got them, and decides to take it. He then falls into a troubled sleep in which he fights all his enemies, while enduring excruciating side pains. The dream ends with George Stacy's spirit imploring Peter to keep protecting New York as Spider-Man, stating that his powers are both a blessing and a curse. When he wakes up, Peter notices a ghastly thing: he has grown four extra arms. The serum has increased his spider powers rather than nullifying them.
After rejecting an invitation to a movie from Gwen, and a late-night photo assignment from the "Daily Bugle", Peter, as Spider-Man, calls Dr. Curt Connors for help, who offers him his summer house as refuge. Elsewhere, Michael Morbius is found stranded in the ocean by a boat and taken aboard, only for numerous passengers to then mysteriously disappear. When the crew confronts him, Morbius attacks them and flees. As night falls and the crew goes to sleep, Morbius, now resembling a vampire, returns and feeds on their blood, but is overwhelmed by guilt and attempts to kill himself by jumping into the ocean. He washes ashore and stumbles upon Connors' summer house, which he enters before falling asleep. Meanwhile, Spider-Man is working on a cure for his condition in Connors' lab in the lower levels of the house and, frustrated, smashes a test tube. Morbius is awakened by the noise and attacks Spider-Man. As the two fight, Connors arrives and the stress of being attacked by Morbius causes him to transform into the Lizard.
Morbius and the Lizard fight over who will kill Spider-Man. Morbius manages to bite the Lizard before escaping. An enzyme in Morbius' blood allows Connors to temporarily regain control of his mind, and he joins forces with Spider-Man to track down Morbius, believing that his blood can help cure them both. Meanwhile, Morbius reflects on how he became a vampire following a failed experiment that he, his fiancée, Martine Bancroft, and his assistant worked on to cure Morbius' rare blood disease; he killed his assistant before jumping into the ocean, not wanting to hurt Martine. As Spider-Man and the Lizard search for Morbius, Gwen and Aunt May worry about Peter, as they haven't heard from him in a while, and J. Jonah Jameson reveals to Robbie Robertson that the "Daily Bugle" is facing financial problems. Eventually, Spider-Man and the Lizard find Morbius and defeat him, but the vampire ends up falling into the river and disappears. Nonetheless, Spider-Man managed to retrieve a blood sample from him, which he uses to cure both himself and the Lizard.
Other versions.
Ultimate Marvel.
In the Ultimate Marvel version of Clone Saga, a clone of Spider-Man has six arms and a variant black costume surfaced. When unmasked, this clone is revealed to also have more spider-like physical features, such as fangs around his mouth, additional eyes and spiky hair along his entire face. The variant cover for "Ultimate Spider-Man" #100 was based on that of "The Amazing Spider-Man" #100, in which the saga takes place.
Mutant X.
In the "Mutant X" universe, Spider-Man still has six arms. For unexplained reasons, he has taken to calling himself Man-Spider instead of Spider-Man. Moreover, the chemical he consumed apparently altered his DNA, since his clone (who is killed by Madelyne Pryor within the series) also has six arms.
Pestilence.
Deadpool encounters a version of Spider-Man in a universe which he refers to as "an Age of Apocalypse" (not "the" Age of Apocalypse). In this reality, a six-armed Spider-Man becomes Pestilence, Horseman of Apocalypse, with poisonous fangs and engages in cannibalism.
What If?
An issue of "What If?" had asked "What If Spider-Man had Kept his Six Arms?". Morbius is killed by sharks (they were attracted to the blood on Morbius when he landed in the water) before a cure for Spider-Man's mutation is found. After a fight with the Lizard, Spider-Man is advised by Dr. Curt Connors to see Professor X. Spider-Man ends up in a scuffle with X-Men members Angel, Beast, Cyclops and Iceman until Professor X and Jean Grey break it up. Professor X then examines him with Cerebro and learns that Spider-Man's mutation is permanent. Spider-Man then visits Mister Fantastic for a second opinion. Mister Fantastic also states that the mutation is permanent. When the Thing comes in announcing that Doctor Octopus is holding hostages at City Hall demanding to see Spider-Man, Spider-Man easily takes down Doctor Octopus with his six arms. Spider-Man gains enormous popularity and is the nationally recognized spokesperson for the physically challenged, thereby proving J. Jonah Jameson wrong in his assumption that Spider-Man would be considered as a freak. Afterwards, Mister Fantastic summons Spider-Man back to present his latest invention...four arm sheaths that will keep the extra arms invisible (as long as they are worn) when Peter Parker is in his civilian outfit.
Years later, Aunt May has died of natural causes and Peter begins his full-time career as a superhero. His extra limbs give him augmented strength and agility. Spider-Man also prevented the death of Gwen Stacy at the hands of the Green Goblin as well as participating in the Secret Wars where his alien costume soon resulted in the creation of Venom whom Spider-Man also defeats. The story ends with Peter accepting the situation in which he himself was responsible for and making the best of it.
Spider-Verse.
In the run-up to "Spider-Verse", after learning that something is crossing dimensions to kill Spider-Men, the Superior Spider-Man (Doctor Octopus' mind in Spider-Man's body) assembles a team of the more ruthless surviving Spider-Men to oppose this threat, including a six-armed Spider-Man wearing a costume perfectly adapted to his condition. During the storyline, this version of Spider-Man accompanies Spider-Man Noir to a world where the spider-bite left Peter in a coma, with the six-armed Spider-Man using the serum that gave him his extra arms to cure Peter of his mutation for good, guessing that Peter's mutation being so new would make it more receptive to the serum. However, the six-armed Spider-Man is later killed during a trip to 2099 with Lady Spider and Spider-Man 2099 when he is caught and fed on by one of the Inheritors.
In other media.
Video games.
The six-armed Spider-Man appears as a playable character in the mobile game "Spider-Man Unlimited".