A hundred years ago, the most recent deity of magic was Mystra, Lady of Mysteries. She was the third incarnation of said deity, being a mortal formerly known as Midnight, who was murdered on her home plane of Dweomerheart by the deity Cyric, Prince of Lies and God of Murder. This had caused the Spellplague, in which magic throughout the realms ceased to function through the Weave, forcing spellcasters everywhere to re-learn how to use it.
A few decades before this event was the ascension of Midnight, during which she replaced the second Mystra who was killed by Helm, God of Duty, during the Time of Troubles. This was an event in which all gods walked the lands as mortals, and was caused by the theft of the Tablets of Fate. When these tablets were returned to the overgod Ao, said overgod bestowed upon Midnight the divinity and portfolio of magic. And so in Mystra’s honor did she assume her old identity.
No, none of these deities could be the “True Lord of Magic” the voice spoke of. But what about the second version of Mystra?
Millenia before the Time of Troubles, the second version of Mystra was born. Her birth came as a contingent result of the destruction of the first, who was then known as Mystryl. Mystryl was the pre-eminent deity in the original kingdom of Netheril. Many more floating cities existed back then, Shade Enclave among them, but the most powerful of all the nation was Karsus enclave.
Karsus was the most powerful of all the arcanists who ruled Netheril, and his folly was his undoing. He had concocted the most powerful spell in history, a spell that, upon its casting, had stolen the divinity from Mystryl. This killed her outright, and the divine power entered the arcanist all at once.
Evidently the strands of the Weave, controllers of all magic, were far too complex for Karsus to understand, resulting in a loss of control and magic everywhere. All but a few of the floating Netherese enclaves plummeted to the ground, resulting in the terrible fall of the great kingdom.
Mystryl, being feminine, couldn’t be the lord of magic reference by the voice, but Karsus…
The realization hit Ezmereth, and he could see in his master’s eyes that they had both come to the same conclusion: this was a temple dedicated to the worship of Karsus, the selfish arcanist whose quest for power had caused much destruction so long ago.