Rise of the Runelords 3.75
Runeforge was created during the reign of Thassilon as a place where agents of the seven runelords could gather to study magic and devlop new spells and magic items. The runelords warded Runeforge against entrance to the complex by any runelord or his direct servants, to keep the research within free from sabotage at the hands of an enemy.
Those who joined Runeforge joined for life, the magic of the complex sustaining them indefinitely without the need to eat, drink or sleep.
The final project the runelords set Runeforge on was the development of ways that the runelords could escape the imminent fall of their empire. It appears that each faction engineered a unique solution to this problem, based upon the underlying principles of their magical traditions.
The physical location of Runeforge was a closely guarded secret, known only by the runelords themselves and a few, trusted confidants. The entrance to Runeforge was hidden way, ensuring that only the most worthy could even hope to enter its halls. Even divination magic cannot find Runeforge, due to its powerful wards and protections.
Because of this secrecy, all knowledge of the location of Runeforge appears to have died with the fall of the Thassilonian Empire and even the few remaining legends have faded completely from memory.