Y'Nfalle: From Beyond Ancient Gates

Millennia ago, the Demon Lord fell, and the world finally knew true peace. Great Mages spread their knowledge of mana to every corner of the world until every nation and every race received their blessings. With no more of the world to discover, the Great Mages erected massive portal gates across the lands to use as portals to the worlds beyond their own. They desired to spread their knowledge of mana to those worlds too, so that another Demon Lord could never rise again and do to other worlds what had been done to theirs. Through the gates they went, and quickly did they return, disappearing from the eyes of the world without ever telling a soul what was it that they witnessed on the other side. The gates remain still, their true purpose lost to time. Although the world mostly forgot the legend of the Mages and what the gates they built were for, the other side did not. That was millennia ago.