Complex sentence: Thus the great irony is that while the Sith can unnaturally keep others alive but not themselves it is in fact the Jedi who have achieved immortality by accepting their own deaths and becoming one with the Force.
Split into simpler sentences: Thus the great irony is that while the Sith can unnaturally keep others alive, but not themselves. It is, in fact, the Jedi who have achieved immortality by accepting their own deaths and becoming one with the Force.