Ssessibil was an archmage not native to Toril living with a small entourage in the Lonely Tower. Often, he was away from the tower in his home in an alternative Prime Material Plane, where the dominant species, like himself, was a race of giant cat-like reptilian bipeds.
Ssessibil, while having very few interests in the Silver Marches, protected his dwelling with the help of fearsome elemental guardians and destructive traps. For him, the Lonely Tower was a secret refuge, a fortress of solitude, where enemies and rivals from his home plane would never have the opportunity to locate him.
During his last absence from the tower, which lasted about thirty years, the wizard Arbane was able to overcome the defenses of the Lonely Tower and, after stealing Ssessibil's treasure, proclaimed himself master of the tower. This lasted until a company of adventurers defeated him in combat, allowing the return of the previous owner.