The Hawkmen are introduced in the Flash Gordon (comic strip)'s second storyline, "The Monsters of Mongo", making their first appearance in the July 8, 1934 strip. The Sky City is used as a "prison city" by Ming the Merciless, who sends Flash Gordon, Dale Arden, and Prince Barin there when he's forced by sacred law to spare Flash's life.
Initially, Vultan is a willing servant of Ming and Princess Aura, who boss him around fiercely; but he soon takes a shine to Dale and begins to resent Ming's influence. He orders Flash and his friends to work as slaves in the Atom Furnaces, which process the radium that the city needs to stay aloft in the sky.
Flash leads a slave revolt, and sets off an explosion in the radium furnaces. This threatens to topple the city out of the sky; but Dr. Hans Zarkov invents a new atomic Light solidifier, which makes the city even more stable than before. Vultan is grateful, setting his captives free and giving Zarkov a job as his head scientist.
In the third story, "The Tournaments of Mongo", Vultan protects Flash and Dale when Ming travels to the Sky City to collect them. Vultan calls for a "Tournament of Death" so that Flash can have a chance to fight for his freedom. When Flash becomes the King of the Cave World in "The Caverns of Mongo", Vultan sends a delegation of Hawkmen to accompany Flash.
The Hawkmen of the 1936 Flash Gordon (serial) follow the original strip's continuity very closely. King Vultan and the Hawkmen first appear in the fifth chapter, "The Destroying Ray", and Vultan immediately sets to menacing Dale and enslaving the rest in the atom furnaces. Flash is tortured by Vultan in "Flaming Torture", and sets off the explosion in the atom furnaces in "Shattering Doom".
Zarkov saves the Sky City in "Tournament of Death", and Vultan befriends the heroes. When Ming arrives, Vultan calls for a "Tournament of Death", although in the serial this tournament is on a much diminished scale -- Flash must fight one Orangopoid, instead of the thousands of warriors in the comic strip's Tournament.
Vultan remains a loyal friend to Flash for the remainder of the serial, even going to prison on Flash's behalf in "The Unseen Peril".
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You had to keep the spirit of the thing, but then update it. But I still want to do, like, the Lion Men. I want to do Hawkmen. But how do you do Hawkmen and not make it stupid and silly? We have Hawkmen, but they're guys that follow the way of the hawk and are all about birds, and they do kind of fly, but they're not guys with wings sprouting out of their backs... The Vultan character is a much more grounded leader of his people.