As a military unit prepares to capture Solomon Grundy, Aquaman intervenes to rescue the behemoth. Aquaman takes him to the tower of Dr. Fate, who is preparing a mystic incantation. Fate reveals that Grundy was a gangster named Cyrus Gold who was killed and dumped into a swamp. Grundy agrees to help them, while the Justice League get word of Aquaman's actions and go to confront him, encountering a giant sea creature en route. Once they arrive in Atlantis, Queen Mera denies everything and the League travels to Fate's Tower, arriving and interrupting the ritual to seal a breach and banish the Old Ones.
Grundy attacks Superman and Aquaman and Wonder Woman fight – Fate banishes them to Egypt and Easter Island respectively so he can complete the ritual. Hawkgirl attacks him and discovers that the ritual uses Thanagarian runes. Superman defeats Solomon Grundy and Aquaman defeats Wonder Woman – upon their return Superman defeats Aquaman as well and the League confronts Fate. As they ask for an explanation a gigantic mystic creature makes its entrance.
Dr. Fate (first appearance - More Fun Comics #45, May 1940 - origin revealed in #67, April 1941) was created by Gardner Fox and originally Kent Nelson, the 12 year old son of aracheologists who were killed while unearthing an ancient sorcerer, Nabu. Nabu taught the boy vast magicks, gave him a golden helm and amulet, and matured him to adulthood, then "died" and apparently merged with Fate's golden helmet. Fate's powers have varied over the years and on occasion he wore the golden helmet without the bottom half. Not one of the more popular Golden Age heroes despite his Justice Society membership, he was brought into the Silver Age as an "Earth-2" hero (as were most of the Golden Age heroes). His popularity grew substantially upon his "return" and he had several mini-series and played a substantial role in various crossovers. It was later revealed that the helmet held the spirit of Nabu, who could take over Nelson and/or make him less human. Through an elaborate series of rewritings and retcons, Nelson later died and has had two or three successors who took on one aspect or another of the "Fate" guise. Currently, Hector Hall, the son of the Golden Age Hawkman, is Dr. Fate and a member in goodstanding of the JSA. The "Dr. Fate" seen in Justice League is the original, Kent Nelson, although how much of the comic book origin is applicable to him is not yet clear.
In the WB animated universe, Dr. Fate first appeared in the Superman: The Animated Series
in the episode "The Hand of Fate"
(along with Inza, see below). Fate's origin is not stated although he and Superman have met before and Fate still has his tower in Salem, etc.
Inza is Inza Cramer-Nelson, first introduced in More Fun Comics #55 along with her future husband. The Lois Lane to Dr. Fate, he ended up rescuing her on several occasions from whoever the evil mystical or scientific villain of the issue was. He revealed his secret identity to her 11 issues later and she aided him on occasion (such as in All-Star #15, when she dressed as Dr. Fate to track down Brain Wave), and the two of them married (in Flash Comics vol. 1, #306). Inza shared in her husband's immortality, but when the spell failed, she went insane and killed herself. Her spirit and Kent's entered into the Amulet of Fate and she seems to have hung out there ever since. She apparently knows some magic, but has never demonstrated the kind of abilities she displays here, so her abilities and origin may be substantially different.