(From the archives of the Geographical Society of the Triune Empire, Aarenis
It was in 1137PC (or 2483SS by the Laurentian reckoning) that King Oland of Varangia made his disastrous assault upon the Univeristy of Linnarson. ....
To this day, none know precisely went wrong, but on the afternoon of the 10th day, a cataclysmic explosion ripped through the Great Library, pulverising the entire edifice. The blast wave echoed forth across the city; none of the buildings close to the Library were destroyed, and few buildings in the city remained undamaged. More devastating though, if that can be possible, were the chaotic magical energies released by the disaster. Any living thing (including plants) within a day’s travel of the city was warped by the energies. Many creatures were so badly warped that they died on the spot, whilst those who survived were changed, often beyond recognition. Some creatures with intelligence lost it, whilst others which had previously had none gained. The Varangians, of course, were all warped, as were all the inhabitants of the city. Within three days ride of the city the effect could be described as severe, and subtler signs of the catastrophe continued even further.
In short, this is a great explanation for a dangerous set of ruins, filled with odd monsters, deep treasure, and strange happenings. There are many reasons why one would need to enter this forsaken place, beyond just sheer greed of goods left behind by the original inhabitants and many of those that have come here before you. Thus there is plenty of motivation for an party of adventurers to enter this place.