The Lost King is a d20 System scenario for 5th- to 7th-level characters. Part of AEG's Adventure Keep line of short, cheap modules in half-sized format (4-1/4"x11"), it is intended to be run as a single session. From the back cover:

Draneken Castle recently withstood a massive invasion by the Barbarians of Beloran Gorge. During the course of the fight, the court wizard, the self-titled Keland the Magnificent, sent His Majesty King Ivol Lareth II into hiding. Shortly afterward, however, Keland died. In fact, he died before telling anyone what he did with King Lareth.

Now, the King's unaccomplished son is ruling over Draneken and King Lareth's trail grows colder. With all of their options exhausted, you are the kingdom's last hope. If you can bring home King Lareth, the wealth and gratitude of an entire kingdom is yours.

Keland, for all his boasting, was still a rather shrewd character as he made finding King Lareth difficult. Your search begins in Keland's chambers in Draneken Castle. Only a fool enters a wizard's chambers without an invitation.