Warren Worthington III, aka Angel, was one of the original members of the X-Men. Although he is one of the founding members of the team, recruited by Professor Charles Xavier himself, Angel is probably best known for his transformation at the hands of Apocalypse into Archangel.
The events leading up to Angel's transformation began with the Mutant Massacre that occurred in the '80s. A group of villains called the Marauders invaded the underground tunnel of the Morlocks (mutants living underneath New York) and began slaughtering every mutant in sight. The event was shocking in that it was one of the most violent events in X-Men history and left a long-lasting effect on the X-men teams involved in the assault.
Angel suffered one of the most brutal injuries of the battle. The Marauder Harpoon threw his trademark spears into Angel's wings and left him pinned against one of the walls of the tunnel.
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As a result of the damage suffered at the hands of Harpoon, Angel's wings were amputated. This sent him into a deep depression, which led him to crash his personal jet in what appeared to be an act of suicide.
However, before he was killed in the explosion of his plane, Warren was taken by Apocalypse. The immortal mutant offered Worthington the opportunity to regain his wings if he agreed to serve Apocalypse as one of his Horsemen. Despondent over all he had lost, Warren agreed.
When Angel was next seen in the pages of X-Factor, he had been completely changed by Apocalypse. His skin was blue and he had razor sharp wings extending out of his back.
Warren's abilities were greatly enhanced as well. Before his transformation, his sole mutant ability was the power of flight, but his new wings were able to cut through almost anything at Warren's command. He could also use the individual feathers of the wings as projectiles that could not only slice through opponents but also temporarily paralyze them by way of a potent chemical that worked as a neural inhibitor. Additionally, his new wings greatly increased his flying speed, allowing him to soar through the air at unprecedented speeds.
This new incarnation of Angel was simply called "Death" and he was the leader of Apocalypse's Horsemen.
Death came into conflict with his former teammates in X-Factor during the Fall of the Mutants. He felt no loyalty toward them, in no small part due to drugs administered to him while Apocalypse experimented on him. Warren's personality changed as well; while he was formerly a happy-go-lucky playboy, Warren became cold and brooding, a persona more in line with his new namesake.
While his former teammates were reluctant to harm him, Death had no such reservations and used the weaknesses of his former friends to defeat them in their initial encounter. However, in a subsequent encounter, Iceman tricked Warren into believing he had killed his former friend and the shock was enough to overcome the effects of the drug and free him from Apocalypse's control.
Once Warren was free and back in his right mind, he still retained aspects of the dark personality he had taken on. He initially refused to rejoin X-Factor, rationalizing that he had changed too much to continue being a hero. He later revisited that decision after a bit of soul-searching (and the decapitation of arch-enemy Cameron Hodge) and joined the team under the new code name "Dark Angel" before deferring to "Archangel" during Inferno.
While Warren has still struggled over the years with his new, darker persona, there is little doubt that the alterations to his powers have made him a greater asset to the X-Men. Since his return, he has held his own with Wolverine and been a part of the mutant black-ops team X-Force; he was even offered leadership of a team of X-Men. The changes to his character have resonated well with fans over the years, making him not only one of the most venerable members of the team but also one of the most popular.