Complex sentence: Doop was thought to have died with the rest of X-Statix on their final mission, but vacationing X-Men Havok and Polaris recently encountered Doop when he crashed to Earth from outer space.
Split into simpler sentences: Doop was thought to have died with the rest of X-Statix on their final mission. However, the vacationing X-Men Havok and Polaris recently encountered a Doop-like entity when it crashed to Earth from outer space.