Complex sentence: For a time Salaak lived in the future filling the identity of Pol Manning as Hal Jordan had before him (and was married to a human woman) but returned to the present day to warn about the destruction of the Corps.
Split into simpler sentences: For a time Salaak lived in the future, filling the identity of Pol Manning as Hal Jordan had before him. Salaak returns to the present day to warn about the destruction of the Corps.