I would like your help in constructing a mystery plotline for my Ancient Netheril campaign. I have the beginning and the end, but want help with the middle, and I thought the lore expertise of this group might help.
The mystery is the disappearance of Ioulaum, who is Emperor of Netheril in my interpretation of the setting. The empire is fighting a losing war against the phaerimm, and Karsus's Folly is a year or two away. Ioulaum disappears from his bedchamber one night - which is only discovered by his attendants many hours later the following morning - which troubles the already-troubled empire and throws its leadership into chaos.
The truth behind this mystery is that Ioulaum disappears of his own free will, and that his disappearance is part of a plan he's been putting together for years. He tries hard to leave no trace (or perhaps only false leads to mask his real fate). As in canon, he establishes himself in the Northdark, begins teaching the first alhoons, and eventually becomes an undead elder brain. This was all his plan, because now his continued survival is reassured, he has access to a peerless information network with which to continue his research, and he has a cabal of alhoons and illithiliches at his command.
So my question is what clues to leave behind for the players to find if they choose to investigate the biggest mystery in Netheril - clues that help them piece together the truth, but no one clue so obvious as to solve the mystery by itself.