The Spectre is a timeless cosmic entity, the avatar of God’s vengeance on Earth. In order for it to dole out divine vengeance in a just and appropriate manner, it must be imbued with a conscience, and the only way this can happen is for it to be permanently bonded with a human host.
The host most commonly associated with the Spectre is Corrigan, a police detective who was selected after he was murdered in the line of duty — although he is not the only character to act as the Spectre’s vessel. Most notably, fallen Green Lantern Hal Jordan once became host to the entity as a form of penance during the 1999-2000 Day of Judgment event, and Gotham City cop Crispus Allen became the Spectre of DC’s New Earth after being murdered by, of all people, Corrigan, who was a crooked cop in that universe.
Whosoever hosts the entity becomes, to put it mildly, insanely powerful. The Spectre is considered to be one of the most formidable characters in all of DC lore; his physical strength has no upper limit, he’s immune to all but the most powerful magical attacks, and he can manipulate time, space and matter at will.
In the 1985-1986 Crisis on Infinite Earths miniseries, the Spectre is the one who unites all of Earth’s heroes and villains against the Anti-Monitor, who plans to destroy the entire multiverse. The villain very nearly succeeds, with only five Earths surviving his onslught; the series’ climactic merging of these five Earths into one is the work of the Spectre, who accomplishes the feat by creating a burst of energy powerful enough to splinter time and space. So… yeah, he’s pretty hardcore.