How was the Tower of High Sorcery cursed?

Many people fear what they don't understand. The Kingpriest of Istar, the capital of Krynn, had become fearful of that which he did not understand, namely, magic. In his fear, he had the public turn against the magic users and have them clear out of their towers. Two towers were destroyed by the mages themselves, not wanting them to fall into hands that didn't understand their purpose, but the last two were in major cities. In Palanthas, upon the mages clearing out, a black robed, or dark mage, killed himself upon a fence around the tower and cursed it until a specific mortal was to grace its gates, no other mortal could come near it.