Shasarak
Dubbed the Wytch-King, Shasarak is an evil former Shianti who has become ruler of much of southern Magnamund at the time of the World of Lone Wolf gamebook series.  This area, known as the Shadakine Empire, was ruled through his ferocious troops, the Shadakine, and the evil Shadakine Wytches.  Shasarak himself was almost destroyed in a fire which occurred in the once-great empire of Taklakot.  Defying the oaths of the Shianti, Shasarak had become ruler of Taklakot and under his rule it had become great, but the people started using dangerous magics which led to their destruction in a conflagration.  Shasarak survived, but at the cost of severe burning; now half his face is permanently covered by an iron mask.

He is a powerful wizard, always wearing a long black robe and carrying a wizard's staff.  He commands evil creatures such as the Kleasa and has even entered into a pact with Agarash the Damned.  Now based in a tightly guarded fortress in the city of Shadaki, he makes his presence known through mental projections, nightmares and his control over his fearsome minions.

Grey Star fights Shasarak at the end of the fourth and final World of Lone Wolf gamebook.  In fact, he is the only real "boss monster" of the entire series.  Having said this, he is not an especially powerful opponent given the extraordinary skills Grey Star has in this fourth book.  When he's defeated by Grey Star, he crawls through the demon gate and joins Agarash in his hell.
Shasarak's minions control all the eastern coastal regions and have subdued the free cities such as Karnali, Forlu and Andui.

It is discovered that Shasarak is a former Shianti towards the end of the second gamebook, when Grey Star uncovers a book bearing a Shianti rune in Gyanima, the last remnant of Taklakot.