A mercurial and ambitious Lastwall captain named Waren Heliu used the keep as his personal magical laboratory. Heliu was a summoner who specialized in fire magic and summoning, and summoned and bound a massive thoqqua, an intelligent worm-like creature from the Plane of Fire. He kept it hidden in a subbasement of the keep but overestimated his ability to control the creature. The thoqqua escaped its wards and began a fire that consumed the keep in flames. Heliu was discharged from the Lastwall military and the ruined keep became known as Heliu's Folly.
The thoqqua Heliu summoned returned to the subbasement under the keep a few years ago. It is currently in a dormant, reproductive state.