"The people suffered greatly within his dark grasp."
"in" sounds more like they're literally inside his hands.
"old body" sounds awkward. "original body" or "age-old" body are more descriptive, and sound better.
"His efforts were futile, as the pair, with the aid of their companions, managed to destroy his body entirely, therefore ensuring his death."
It may seem weird, with the commas, but the ending you had before was choppy. Mainly the "ensuring he could never again return".
Overall, it was really nicely written, though.