The Dimension of Ra was first visited upon by Mark Merlin when he was ambushed at home by an old enemy, the Gargoyle. Mark shrunk himself and then vanished into the dimensional realm known as Ra. There, he met the Egyptian scientist Kranak and his daughter Rimah.
Merlin wanted to return to Earth, but Kranak told him he couldn't -- at least not as Mark Merlin. His body and soul would have to remain in the realm of Ra, otherwise he would die. However, Kranak suggested that Merlin's memories could be reincarnated as "Prince Ra-Man," a young Egyptian sun-god who died accidentally years ago. Agreeing to this, Merlin absorbed the rays of a potion provided by Kranak and he successfully returned to Earth with the body of the Egyptian prince and the memories of Mark Merlin.
Prince Ra-Man had a short stint as a supernatural hero on Earth, but he was able to return to Ra on several occasions.
It should also be noted, that when Zatanna went looking for her father’s books of knowledge, her trans-dimensional journey into magical realms gave her a chance to meet the newly dubbed "King Ra-man."