Relto was Written by Yeesha as a gift to her parents Atrus and Catherine. It was meant to resemble the Age of Myst with Yeesha herself commenting that she placed the library in the same location as it would have been on Myst.
It is unknown when in her life Relto was written but it must have been before she started along the path that lead to The Grower. This can be ascertained by the recorded messages she left behind where she demonstrated a loathing towards the Age of Myst and so would be unlikely to create an Age in the image of a place she despised. Indeed it is likely that Relto was Written before she turned sixteen which is when she left Tomahna to head into D'ni.
First mention of Relto came on December third 2002 by Dr. Marie Sutherland after she witnessed some of the 'Uru Explorers' arriving in D'ni via Jeff Zandi.