Carnage

Carnage is the combination Cletus Kasady and the Carnage Symbiote.

Creation
While traveling on the John Jameson Space Probe 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.

Fighting Spider-Man
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.

Dorammmu's Servant
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 Terri 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 energy 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.

Trapped
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.

Counter-Earth
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.

Personality
Cletus is criminally insane, merging with the symbiote simply made him more so. He acts without any regard towards the people involved. Instead, he thinks only of his personal pleasure.

Kasady hates figures of authority. He saw Terri Lee's arrest of him as a personal attack and went after her when he got powers. He hated Spider-Man for the same reason. He resented his "symbiotic father" Venom for trying to tell him what to do. He accepted Dormammu's demands as it allowed him to act however he desired.

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.

Role in Ultima
In Ultima, Carnage was found in the Dark Dimension by several of the Organization's occultist members, who rescued him in exchange for him joining the Organization as one of their soldiers. Carnage agreed, and was allowed by Padro Lodo's magic to actually kill people while wearing the symbiote suit instead of just merely draining their life energy. However, Kassidy had to remove a piece of his symbiote by order of Lodo in order for the Organization's head scientists to inspect it to see if it and the Venom symbiote can be used to create an army of symbiote soldiers for the Organization's army.