The age of the High Elves is marked by nearly unquestioned mastery over the planet due to the Elves' being the sole masters of magic. This mastery allowed the High Elves to build cities, cultures, and temples still unmatched today, as well as allowing the elves to enslave entire races before creating Humans to fill the need for physical labor and other menial work.
Eventually the abuse of Eridena's magical resources would deplete them, leaving the High Elves unable to hold onto their power and control over the other races. Rebellions begin en masse once this becomes common knowledge, and the entirety of High Elf society is brought to its knees.
