Complex sentence: Following that group's defeat he reformed and unbeknownst to the X-Men was recruited by Professor X to replace him while he isolated himself to prepare for a defense against the alien Z'Nox invasion.
Split into simpler sentences: Following that group's defeat he reforms and unbeknownst to the X-Men is recruited by Professor X to replace him. The Professor desires to isolate himself to prepare for a defense against the alien Z'Nox invasion.