Complex sentence: The Source incarcerated him, most likely because Zankou is the most powerful magical being in existence a half demon, impossible to vanquish, and can best The Source, the Charmed one's in strength and power.
Split into simpler sentences: The Source incarcerated him, most likely because Zankou is the most powerful magical being in existence. He is a half-demon, impossible to vanquish, and can best The Source and the Charmed one's in strength and power.