"Should a man of royal blood spend a night upon it, he would either receive wounds and blows or see a marvel."―Henry Jones, Sr.[src]
Gorsedd Arbeth (also spelled Gorsedd Arberth) was the name of a hill in rural Wales with a connection to Celtic lore. One side of the hill had a cliff overlooking a lake. In ancient times, Celts deposited a trove of weapons and armor into the lake, perhaps as a sacrifice of their war booty to the deity of the lake.
In March 1945, while escaping from Nazis on the trail of the Spear of Longinus, Henry Jones, Sr. recounted the lore of the hill in an effort to impress their getaway driver, Rebecca Stein. While highlighting its royal connection, Jones was interrupted by Brendan O'Neal, who claimed his own royal connections to impress Stein.
After a flat tire caused the party to stop somewhere in Wales, Indiana Jones climbed a nearby hill to get a small nap. In his sleep, he received a vision from the mysterious blond woman, who revealed that he had been chosen to help protect the Spear, but first he would suffer the blows and wounds, and that his freedom was would lie beneath the water.
Jones awoke, to a beating by the Nazi goons, Kurt and Jorge. His momentary advantage on Kurt was stopped by Dieterhoffmann's drawn pistol. The Nazi agent reclaimed the half of the spear tip from Jones, and had him pummeled some more, before being bound. With his father and Stein in captivity, Jones connected Dieterhoffmann's plan to finish assembling the spear at Connely's Inn in Ireland -- before being pushed off the cliff into the lake, with a large boulder tied to his feet.
Believing Jones finished, the Nazis and their captives left for Holyhead, shooting out the tires of Jones' car. Underwater, Jones found the ancient Celtic treasure, and used a sharp weapon point to cut his ropes. Grabbing the weapon tip as a souvenir to remind him of this archaeological find, he surfaced, and was pulled to shore by the escaped O'Neal. Without a working car, they resolved to find alternate transportation to Holyhead - and hopefully not a horse cart.
Sometime after the quest for the Spear of Longinus was completed, but before August 1945, Jones returned to find the lake with the weapons trove, but after exploring many lakes and hills, left empty-handed.