By 1999, Spider-Man's origin story was common knowledge; Peter Parker was a high school student bitten by a radioactive spider that in turn gave him the powers of a spider, including enhanced strength, a spider-sense that warned him of danger and the ability to stick to walls. But in 2001's Amazing Spider-Man #30-35 by J. Michael Straczynski and John Romita Jr., everything previously known about the web-slinger's origin was changed, due to the introduction of two men who blew Spider-Man's world apart, Ezekiel Sims and the evil Morlun.
The issues started with Peter Parker in an existential crisis. After Mary Jane had left, Peter was considering taking on a teaching job at his old high school. However, things got worse when Ezekiel revealed to Spider-Man that it wasn't a radioactive spider that gave him powers, but a spider that then died from radioactive poisoning. Instead, the spider was merely a messenger from the spider-totem, which chose Peter Parker to become Spider-Man.
When Morlun was introduced, he arrived in Manhattan by boat, hidden away, before he started to prey on people. He could feed on anyone, but he needed to feed on a pure member of an animal totem to power up completely. He was there to find Spider-Man, knowing that sucking all the life-force out of a person who has a pure connection to the spider-totem could give him 100 more years of life before he needed to feed on something that powerful again. Even worse, the vampiric being was far more powerful than Spider-Man, as he aptly demonstrated throughout these issues.
Morlun's main power is to absorb the life force from a person and take it into himself. He is part of the Inheritors, a family that has been feeding on people for many years. He has superhuman strength, speed, stamina, agility, durability, and reflexes. He can do everything Spider-Man can do, and he can do it better and longer.
In Amazing Spider-Man #35, Spider-Man was alone in the fight. He believed Ezekiel had died saving him, and he had no way of beating Morlun in a one-on-one fight. That is when Peter Parker created the same type of radiation that infected and killed the spider that bit him and then injected himself with enough to weaken him but not kill him. He knew that once Morlun tried to take his life-force, he would absorb the radiation, and it worked perfectly. Morlun was injured severely. The radiation burned inside of Spider-Man, but it was devastating to Morlun, who couldn't handle the energy, and it began to drain his life force. With Morlun weakened, his familiar, Dex, shot him.
However, Morlun was not dead. He returned later with his family, The Inheritors, and it took a number of Spider-Men from alternate universes to contain them in the original Spider-Verse crossover. It wasn't until the Spider-Men had the help of Superior Octopus that they were able to actually defeat them, killing the father and saving Morlun's siblings by reverting them into babies. Once again, Morlun survived. And now, Sin-Eater is trying to lure him out to cleanse him and steal his powers, but considering the way Morlun has manhandled Spider-Man in the past, Sin-Eater may be biting off more than he can chew.