Complex sentence: Elminster jumped into the Nine Hells and sealed the rift from there, but that act also burned all his magical energies and left him helpless in the Hells; only with the help of the Simbul did the archmage finally escape.
Split into simpler sentences: Elminster jumped into the Nine Hells and sealed the rift from there, but such an act also burned all his magical energies and left him entirely helpless in the Hells. Only with the help of the Simbul did the archmage finally escape.