Deaths often play a huge part in comic book storylines. They sometimes increase the shock value of a story and other times, act as a motivating factor for the protagonist. Usually, comic books end up resurrecting their dead characters and it gets pretty hard to take them seriously. Sometimes, however, the dead stay dead and greatly affect everything. Here are five such heart-breaking deaths. Before Carol Danvers, Mar-Vell donned the mantle of Captain Marvel. While battling Nitro, he got exposed to deadly nerve gas, Compound 13. Though he defeated the enemy, his body later developed cancer. The brave hero bade farewell to readers in 1982 when he succumbed to the disease, surrounded by comrades and rivals. It was one of the first major superhero deaths in Marvel Comics. Captain America has had multiple sidekicks and partners including Falcon and Bucky Barnes. In one story, Jack Monroe replaced Bucky. His was a sad story where just like other Winter Soldiers, he was abandoned in a cryogenic state and became mentally unstable. He eventually became Nomad and fought to protect the weak, but was later murdered by Bucky in a tragic turn of events. After being possessed by the Phoenix Force, Jean Grey changed for the worst. She became exponentially powerful and the evil within her grew so much that she ended up killing billions! Jean was brought to trial and in a moment of redemption, sacrificed herself to save innocents and get rid of the Phoenix force. It was later established that Dark-Phoenix wasn't the real Jean. Gwen Stacy's death was the second turning point in Peter Parker's life (the first being Uncle Ben's death) that ended up changing his personality. She died after being thrown off a building by Norman Osborn, the Green Goblin. Peter became mature after the incident and Gwen of his timeline stayed dead. In another timeline, she is alive and known as Spider-Gwen. Eric Masterson replaced Thor for a brief duration and did a decent job. When Thor returned, Eric returned his hammer (Mjolnir) and was granted an enchanted weapon called Thunderstrike by Odin. The Bloodaxe, however, turned out to be a corrupting influence. Eric tried his best to stay out of its control, but eventually fell victim to the weapon and died a tragic death.