What If... #4 (1989)
When Spider-Man returned from the Secret Wars, he had what appeared to be a costume made out of an amazing material. When it wouldn't get off of him, the Black Cat said he should visit Reed Richards of the Fantastic Four. He went to visit Reed, but he was not in town. So he went with his friend, Dr. Curt Connors (aka the Lizard), who performed a CAT scan on Spider-Man. The tests revealed that there was something strange in Spider-Man's blood, so he went to Reed Richards again. But the alien symbiote costume had now completely bonded with Spider-Man. Whether the CAT scan had somehow sped up the bonding or if he simply waited too long was not revealed.
Mister Fantastic then imprisoned Spidey in a cell. Even with Dr. Strange's help, Mister Fantastic could find no way to help Spider-Man. Spider-Man escaped, causing chaos everywhere. The Black Cat realized what has happened.
A few days later, the Avengers were in a battle with the Hulk. The symbiote joined with the Hulk, leaving behind an elderly, frail man- Peter Parker! The symbiote had been robbing him of his energy, feeding on it. Peter visites his elderly Aunt May to say goodbye. A few days later, Mister Fantastic found Peter in his lab, dead- of old age. A solemn group attended his funeral including the Fantastic Four, the Avengers, the Black Cat, and even the Kingpin. Apparently, the Kingpin had a deep respect for his nemesis.
Mister Fantastic created a weapon that could kill the symbiote based on his and Peter's notes. All the super-heroes followed the symbiote, and the Hulk, to Mt. Rushmore, where it bonded with Thor. Black Bolt of the Inhumans defeated him, since the symbiote was vulnerable to sonics. They then discussed what to do with the symbiote, when the Black Cat showed up and kills it with a weapon similar to that of Mister Fantastic's. She had photocopied all their notes, and the Kingpin helped her build it by hiring scientists. In return, she agreed to work for him- forever.