Originally, Zadkiel was an angel of Heaven who sided with Lucifer in his rebellion. Before the rebellion ended, Zadkiel betrayed Lucifer and aided in banishing him from Heaven. As punishment for siding with Lucifer, Zadkiel and his black legion was denied entrance to the high heavens and placed in charge of Heaven's black ops. His mission included combating Hell's incursions on Earth and keeping humanity's evil in check. One way he did this was through controlling the spirits of vengeance. He manipulated the spirits to purposely hide their true origin, to keep Heaven's involvement with them a secret. He did this by meddling with their memories and having them take on the appearance of either demons or beings from other religions. Eventually, he grew tired of his role and launched a second rebellion against Heaven, seeking the throne for himself. To accomplish this, he began recruiting human followers on Earth to serve him on the mortal plane and fight for him after they died.
At some point, whether before his rebellion began or after, he answered Roxanne's prayer and bonded a spirit of vengeance to Johnny Blaze to save him from Hell. Being an angel who had not fallen, the Ghost Rider could not be kept in Hell and, by extension, Johnny.
During the time Lucifer escaped onto Earth, two angels who had initially sided with Zadkiel betrayed him and eventually revealed his involvement in the Ghost Rider's creation. The Ghost Rider now seeks vengeance on him for all the pain he has experienced over the years.
To end his stalemate with Heaven, he sought the Ghost Riders' power to break through Heaven's wall. He tricked Daniel Ketch into believing the Ghost Rider endowments were making their hosts unstable, and sent him on a mission to gather their powers & relieve them of their "curse." He also provides Ketch with some of his fallen angels, as well as some of his old enemies as Ghost Rider, to help ensure that he completes his mission. He collects all of the powers of the Ghost Riders given by Daniel Ketch and uses it to open the gate. Zadkiel finally claims the throne of Heaven and grows ever-powerful the longer he is on it. He now seeks to murder the Anti-Christ to prevent Biblical prophecies from coming true and rewrite the future according to his wishes.
This plan was thwarted by a legion of Ghost Riders, who stormed the gates of Heaven and defeated him, saving the Anti-Christ. After Zadkiel is reminded by Blaze that, no matter what throne he seemingly claimed or what power he was able to siphon temporarily, he was still not the one true God. The Creator returns and banishes Zadkiel to Hell for all eternity, where he would suffer torment from a pleased Lucifer forever and ever.
The full extent of Zadkiel's powers stands unrevealed. He is presumably immortal and can travel between Heaven and Earth, if not other dimensions. He can also communicate with beings on the mortal plane through various hosts such as birds.
Zadkiel is a skilled swordsman, able to single-handedly slaughter an entire group of Ashuras (assassins of heaven) with his ebony blade.