The Unseen Terror is a shapeless and formless manifestation of evil.
Many centuries ago, this Terror was disturbed from millenia of sleep. It was so powerful that it required the combined wisdom of all the leading enchanters of that age to conquer it. A book of legends found in the Largoneth Library tells how these enchanters lured the Terror "to a recess deep within the earth" by placing there a powerful spell scroll. When it had reached the scroll, the enchanters trapped it there with a spell that encased it in the living rock. The Terror was so horrible that none would dare speak of it, and because of this, many years later this tale was considered to be quite fanciful. No other chroniclers of the age mention the Terror in any form.
However, in the 940s GUE, a novice enchanter exploring Largoneth with the intention of defeating Krill found the location of the Terror's prison underneath the Largoneth dungeon. This enchanter managed to retrieve the GUNCHO spell that had lured the Terror underground, while still keeping the Terror entrapped. It is known that during the final hours of the confrontation, Krill pondered the idea of using the Terror for his own purposes, but the plan came to naught.