Another prequel volume as you may have guessed from the number.
Probably best read after volume 1 and 2.
More unified than the Origin of the PCs, since it's tied more into the plot, and not into the sorts of random backstories that a herd of PCs may present to a DM.
But we see Xykon's childhood and beginning, and Redcloak's, including why that's his name. And how they met, and how Xykon became a liche, and Redcloak's brother leaving to raise a family and what came of that.
Eugene Greenhilt shows up. So do some other characters mentioned in the main story.
As you might guess from a story about villains getting started, the ending is not happy. Still, it explains a lot.