Complex sentence: He re-appears in Mordavia, serving his Dark Master, later revealed to be Katrina, the first human (in reality, a nosferatu) the Hero sees in Mordavia, and acting mysteriously, she plays with him long enough to awaken the Dark One, Avoozl.
Split into simpler sentences: He re-appears in Mordavia, serving his Dark Master, later revealed to be Katrina, the first person who the Hero sees in Mordavia. Acting mysteriously, Katrina plays with him long enough to awaken the Dark One, Avoozl.