Complex sentence: With his life rapidly fading, Vale surprises Revok by setting his own body alight and then, by a technique unknown even to Revok, Vale transfers his own mind into Revok's body, leaving Revok in Vale's.
Split into simpler sentences: With his life rapidly fading, Vale surprises Revok by setting himself alight. Then, through a technique unknown even to Revok, Vale transfers his own mind into Revok's body, leaving Revok's mind in Vale's.