"The Great Magic"

The Great Magic is an event that swept across the area now known as Tellarus and the Eastlands. They say it occurred about 800 to 500 years ago (127 to 427 ce, somewhere in there...), but the event left few decipherable clues as to its nature, timeline, effect, or events. All that is known is the residue (of many, many kinds) that are left among the ruins and extinct cultures of the land. The duration of the Great Magic is estimated at 100 years or so in most areas, as acient places hold a trace that can be measured faintly by powerful detections, by great wizards.
The Great Magic did leave behind strange things, sometimes boons and often bane, encounted in ruins (including the Gutter of Tellarus especially, but also the ancient ruins of culutres in the area. Some claim to have found evidence that the Great Magic led to the demise of some cultures at the time.
There are certain power-hungry, adventuresome, or thrill-seeking people that brave the dangerous of the ruins, hoping to discover traces of this Magic, and reap any reward that might be there. Few come back, and they often report their frustration in not finding what they sought. Those that do find evidence and power of the Great Magic usually do not come back, for different reasons.