Spider-Man has always had a tumultuous and stressful relationship with the symbiote known as Venom. First wearing the symbiote himself after discovering it during the original Secret Wars event, Spider-Man ended up taking it back to Earth and ditching it after discovering the risks. However, what if Spider-Man had kept the symbiote? What would have happened to him? Thanks to Marvel's What If?... comics, fans get to see the answer, and it's pretty gruesome.
Spider-Man first came into contact with the symbiote suit during the original Secret Wars story. After discovering its ability to augment his powers and strength, Peter Parker determines to take it back home to Earth once Secret Wars concluded. Over time, the symbiote altered and changed Peter's very personality, making him darker and more aggressive the more he wore the suit. Peter eventually saw the risk and dangers of wearing the suit, opting to abandon the symbiote entirely.
The symbiote would then go on to bond with several other hosts such as Eddie Brock (the symbiote's favorite), Flash Thompson, and Mac Gargan to name a few. While Venom would become a frequent enemy of Spider-Man, Venom has of late been more of a lethal protector and has most recently even been considered by Captain America to join the Avengers after Venom's help in stopping Carnage during Absolute Carnage. However, in What If?... #114, the heroes and villains involved in Secret Wars never make it back to Earth, and wind up trapped for 25 years on Battleworld -- including Spider-Man, who has been wearing the symbiote the entire time.
While at a dinner party, some of those present note that they haven't seen Peter Parker eat in weeks, or even years. They assume that it's the symbiote providing Peter with sustenance, no longer needing actual food to live. This couldn't be farther from the terrible truth. When Doctor Doom's son attempts to get back to Earth to invade and conquer it, a battle ensues involving the original Avengers as well as their children. During the fight, Klaw uses a sonic blast on Spider-Man... revealing the true nature and result of the 25 years of symbiosis.
Yes, the symbiote has been slowly eating Peter Parker, right down to the bone. Peter Parker is no more, and the symbiote is merely driving his skeleton around like a puppet. What's odd is that this gruesome fate for Spider-Man isn't really explored in the rest of the issue, especially because it seems to warrant an issue all on its own. The other heroes, like Human Torch who saves Spider-Man from Klaw's attack, don't really even address the fact that their friend and ally has been eaten. The symbiote most likely began eating Peter due to the fact that their bond wasn't a perfect symbiosis like the one it has with Eddie Brock.
It's certainly a good thing that Spider-Man got rid of the symbiote when he did in the actual Marvel continuity, as fans can definitely see what would have been an awful end for the hero.