The Chronicles of GrimmGale & Co.
The Abyssal Gate has existed for many years. It was constructed in a hidden location deep in the Krond Height Mountain Range. With knowledge of the gates location, and also the Crystal of the Abyss to power it, a dedicated mage could open a Gate to Hell, and maintain it indefinitely.
Unfortunately, since it was well hidden, its location can only be discovered in one of many books in the Secret Library of Has.