The Great Eliezer was a large monument to an ancient warrior of the Melki Empire, named Eliezer. This monument was located north of Stahurt, more precisely in some very wild mountains, whose woods were so dense and the terrain so treacherous that few ventured to visit the monument.
Since Luke and his present companions had not crossed the Acronym Plains in the direction of Stahurt, they had not had the opportunity to pass near these mountains, so this was a totally new area for them. Nevertheless, Luke had no regrets about not going straight to Stahurt as Scales had asked him to do before they could have a chance to talk further. If he had gone on to Stahurt at that time, he might have missed the chance to get the information he obtained in New Yukiat.
When Wanney was asked by Luke what the Great Eliezer was, he answered expertly. As a child Wanney was a boy fascinated by stories and tales, so he certainly knew the warrior Eliezer.