Complex sentence: During Ilyana's seven years in Limbo Belasco took her as his heir and apprentice, until one day she defeated him, becoming Limbo's new ruler-and S'ym's master-before returning to the X-Men.
Split into simpler sentences: During Ilyana's seven years in Limbo, Belasco takes her as his heir and apprentice. She ultimately defeats him, becoming Limbo's new ruler-and S'ym's master-before returning to the X-Men.