The Once and Future King

When England is in its darkest time Arthur the once and future king will return from Avalon to save the land.

— a line from the prophecy surrounding King Arthur

The tales surrounding "The once and future king" were rumoured to be the accounts of the Legendary King Arthur of the Britons and his knights of the Round Table. Many believe the accounts to be based either before the Roman invasion or shortly after Rome pulled out of England. With the Lady of the Lake a central character in some of the stories, purportedly living in the lake at the base of Glastonbury Tor.
There have been some who believed the island of Hybrasil is the land of Avalon. Depending on the alignment of the stars, there is a bridge which traverses from the lake to the Isle.
Many believe the myth to be false and only offered a thin thread of hope, since England sank beneath the sea.
There have been rumours coming from sailors of seeing an island rising out of the ocean on starry nights in the north Atlantic just beyond where England once stood proudly. They say they can hear singing in a strange tongue terminating from the island. There have even been tales of ships landing on the island never to be seen again.
Some believe the sirens of old have return to torment sailors and take them to their watery doom. Others have said that when the moon is full and the island appears they saw mermaids and selkies swimming alongside their vessels. Sailors being a superstitious lot have started avoiding the northern passage for this reason.
If this myth is to be believed many say it ties into the rumours regarding the Tower of London.

In Literature

The first time it was fully written out was in "Le Morte d'Artur" by Thomas Mallory, published in 1485 CE. From there it became more widely known throughout the world. By the twenty-first century CE, it had many re-iterations with other people writing their own takes one the legend.