Carnage

Carnage is the combination Cletus Kasady and Venom's symbiote offspring.

Spider-Man
While traveling on the John Jameson Rocket the Venom symbiote spawned an offspring. Dormammu got his assistant Baron Mordo to divert the rocket so that it would crash on Earth returning the two symbiotes. Upon their arrival on the planet the two symbiotes attach themselves to a nearby couple.

Disguised as the young man and under the control of Mordo, the second symbiote entered into Ravencroft Institute for the Criminally Insane after the escape of the newly reformed Venom. The symbiote then removed itself from the anonymous man and joined with recently incarcerated Cletus Kasady to form Carnage.

Carnage then goes to the Stark Industries demonstration to assist his "father" in stealing dimensional portal technology. While Venom becomes distracted Carnage is able to defeat both Spider-Man and War Machine.

Carnage is about to off Spider-Man when Venom stops him. He wants the webslinger for himself. The two begin fighting when Baron Mordo appears to stop them. Carnage grabs the interdimensional probe and the two escape.

After bringing the dimensional technology to Mordo, Venom and Carnage restart their fight. Dormammu stops the fight and tells the two that he in order to enter their world they need an equal amount of life energy to put in his world to replace him. Venom refuses and leaves. Dormammu lets him since Carnage will be sufficient for this new assignment.

Dormammu gives Carnage the ability to absorb the lifeforce from others so that it can be collected and Dormammu can emerge from his dimension. Carnage attacks the NYPD. Spider-Man arrives to stop him and Carnage tries it on him but finds that he's "full" and needs to drain before absorbing any others. He returns to the lair and impatiently drains himself of all the energy he's collected so far. The urn that is collecting all the energy is almost full. It just needs a few more lives before it is ready, much to the chagrin of Carnage.

Madame Web then shows Spider-Man that Kasady is waiting for a "mutual acquaintance" and reminds him to "keep your friends close and your enemies closer." Detective Lee comes home to find Kasady, now Carnage, waiting for her. Carnage attacks the detective and Spider-Man swings in to stop him. Dr. Kafka receives a call that Kasady's been spotted. Dr. Conners warns her to stay put but she must since Kasady is also her patient. When Carnage gains the upper hand over Spider-Man Detective Lee tries to stop him but gets her life energy absorbed. Spider-Man tackles Carnage and the two go off the room and Carnage takes the brunt of the fall. Dr. Kafka arrives and Carnage takes her life engery since it is the last one he needs and escapes.

The urn is now full and the portal to Dormammu's dimension is open. Spider-Man, having teamed up with Iron Man and Venom, arrive before Carnage can release the urn into the portal. Carnage distracts Venom with Dr. Kafka and opens the urn releasing Dormammu. Knowing how drastically overpowered the three heroes are Spider-Man hatches a plan requiring that Venom and Iron Man hold off the three villains without him.

Spider-Man reverses the polarity of the probe and sucks Dormammu back to where he came from. As Dormammu gets sucked back the life energy he took is released into each person, such as Detective Lee and Dr. Kafka. Dr. Kafka awakes and thanks Eddie for saving her, but now the symbiote is taking over again and becomes Venom. Dormammu fully reenters the portal but Spider-Man is unable to shut down the probe. Dormammu created some kind of link between he and Carnage that won't break therefore keeping the portal open. Venom throws Carnage into the portal but Carnage quickly grabs Dr. Kafka. Venom saves her by sacrificing himself to stop Carnage.

Spider-Man Unlimited
At some point both Carnage and Venom escape Dormammu's dimension and team up. The two also gain new abilities and the desire to control everyone. Carnage and Venom board the John Jameson's shuttle to Counter-Earth despite Spider-Man's efforts. While there the two attempt to take over Counter-Earth with an invasion of symbiotes. Spider-Man then travels to Counter-Earth and joins a band of freedom fighters to fight the High Evolutionary, along with the occasional battle with Venom and Carnage.

Powers
Because Carnage's symbiote spawned from the Venom symbiote Carnage has many of the abilities of Venom, though not all. Carnage is agile and strong, though he cannot form webbing. Instead he extends tendrils of himself to act like webbing.

Carnage's most distinguishing trait is how fully Kasady bonds with the symbiote. While Venom refers to himself as "we" Carnage refers to himself as I. Instead of two creatures working closely together as in Venom's case Carnage works as one, single being.

After escaping from Dormammu's dimension Carnage gains the ability to change into a somewhat liquid form. His appearance changes to appear taller, skinnier, and with boney protrusions. How exactly Carnage and Venom gained new powers is unknown. It is possible that Dormammu gave them his new powers or they learned it at some point.

Carnage does have his weaknesses. Like Venom he is vulnerable to extreme heat and sound.

Alternate Versions
In an parallel universe a Spider-Man had angrily attacked his clone. At that moment a Carnage symbiote from yet another universe attached itself to that Spider-Man and formed Spider-Carnage. Spider-Carnage destroyed New York City and was about to destroy every single dimension. However, Spider-Man, along with other Spider-Men from other universes stopped him. Spider-Carnage fled into another universe. This time Spider-Carnage could only destroy one dimension at a time. He captured Spider-Man and imitated that universes Spider-Man. Gwen Stacy, that Spider-Man's girlfriend, released Spider-Man but was captured by Spider-Carnage. Spider-Carnage was about to destroy that dimension when Spider-Man brought that universe's Uncle Ben to him, this version had not been killed. Uncle Ben reminded him about great power and great responsibility. Peter then tried to reject the Carnage symbiote but was unable to. With no other option Peter threw himself into a portal and destroyed him and the symbiote.

In another timeline Spider-Carnage was able to destroy all of existence. However, when the blast reached the Beyonder's dimension he stopped and reversed time so that he and his assistant could train other Spider-Men to fight and stop Spider-Carnage.

Background
A proposed sixth season of Spider-Man would have seen Spider-Man going England to find the lost Mary Jane Watson. Meanwhile, Carnage would be there and have become Jack the Ripper. However the series ended and no sixth season was ever made.

It is implied, though never directly said, that Spider-Carnage formed from this universe's Carnage symbiote. But since Carnage made a return in Spider-Man Unlimited it can be assumed that it is a different Carnage symbiote.

In Spider-Man Kasady was incarcerated with Eddie and became Carnage in Ravencroft. While in Spider-Man Unlimited he was imprisoned with Venom and became Carnage in S.H.I.E.L.D. How this is explained in universe is unknown.

Kasady in the comics was a homicidal murderer. When merged with the symbiote he became even deadlier. But as this is a show meant for children none of these aspects would have been allowed. Carnage is still made to be dangerous. Robbie Robertson does state in Venom Returns that Kasady has done things "even the Post wouldn't report" implying that he may have done something the censors would not have allowed such as murder. Carnage was still able to form his iconic hand knives and hand ax though he was unable to actually use them.