Looks like artist Gary Gianni and writer Mark Schultz have been having a bit of fun with Prince Valiant. A few months go, a mysterious stranger appeared in a blinding flash of light, apparently dumped from the sky. He had no memory, but Aleta recognized him.
When she had been held in a magic-induced coma by the sorceress Maldubh, Aleta had floated through an ethereal nightmare world, and had been saved by the stranger there. The stranger says that he had been torn from the grasp of his love, but beyond that, he remembers nothing.
The stranger (dubbed St. George by a somewhat jealous Prince Valiant) proves to be a strong fighter. He accompanies Prince Valiant, Aleta, and Sir Gawain on a quest to hunt down and destroy a golem (they don't call this an adventure strip for nothing).
Unfortunately I don't have access to the earlier strips in this sequence, but look closely at the stranger's clothes in the panels below. They give the savvy comics reader a good clue to his identity. One blond-haired hero of a Sunday-only strip is usually depicted wearing a red shirt with a sunburst and trousers tucked into boots. (click on images to enlarge)
Still not sure? There's a telling clue in his final appearance... "But then a bolt of lightning strikes, and the bold warrior disappears -- in a flash."
The unknown hero was, of course, Flash Gordon. Around the time he appeared in Prince Valiant, this was happening in his own comic strip...
Sadly, Flash Gordon has been in reruns since 2003 so there was clearly no collaboration between the two creative teams.
But it's still a nice tribute from one legendary strip to another.