This is a story about faithful men who were sealed up in a cave by an emperor who persecuted their faith. They didn't die; instead, they slept for 200 years, awakening during the rule of an emperor who shared their faith... but they had no idea at first that they had been asleep that long, kind of like Rip van Winkle. The story is popular in both the Christian and Muslim traditions.
King in the mountain.
The Seven Sleepers article linked that story to the motif of the "king in the mountain" which is a common type of folktale in which a king or hero sleeps inside a mountain, waiting to return and rescue his nation in a time of danger. There is a HUGE list of the kings and heroes who appear in this type of legend; I know this story is told about King Arthur (in fact, it's in the UnTextbook: Arthur in the Cave), but there were so many other kings and heroes here. I'm having a hard time deciding which one to click on and follow.
Gerald FitzGerald, 3rd Earl of Desmond
Of the many heroes, I chose this one: Gerald FitzGerald was an Irish earl who lived in the 14th century, and some of his poetry has survived (he wrote poetry when he was imprisoned as a result of a violent struggle with a rival, Brian O'Brian), but he is more famous for the legends told about him. He supposedly had an affair with the goddess Aine, and when he disappeared in the year 1398, a legend told that he was sleeping in a cave near (or under) the lake of Lough Gur in County Limerick, Ireland, and that someday he will ride forth again on a silver-shod horse; in some legends, he appears on the banks of the lake every seven years.
(Lough Gur, where Gerald is sleeping)
She is an Irish goddess of summer and of prosperity; she is also a love goddess and is considered to be "Queen of the Fairies" in County Limerick. In some versions of her encounter with Gerald, he raped her and, to punish him, she turned him into a goose or, in some versions, she killed him. Knockainy Hill in County Limerick is named after her: Knock-Ainy, The Hill of Aine: