Everything about the plague, really. It's a fusion of H1N1 and Marburg, which would not have been possible without magic (since they're two very different sorts of virus). It's 100 percent fatal; once symptoms show, death usually comes within two to three days, during which time the victim is delirious with fever and bleeding from every orifice. It wipes out most of society in less than a month. Parts of several chapters are devoted to it in lovingly nauseating detail, along with the memories of the few survivors who find New Lasgalen (or are found by the Elves).
The architect of the plague, Avathar, is for much of the story walking Nightmare Fuel in his own right. The sole fallen Maia to escape the destruction of Angband, he found a volcano and took a snooze for around eighteen millennia. When he woke, he adapted to the human world very fast via Mind Rape, and spent the next two centuries shaping and manipulating the world (including the rise and fall of Nazi Germany). He creates the plague out of essentially pettiness.
