Complex sentence: They are able to call upon it for magical armor and powers to fight superhuman evildoers; although, once their strength is exhausted they must rest in their sarcophagi to regain the ability.
Split into simpler sentences: They are able to call upon it for magical armor and powers to fight superhuman evildoers. Although, once their strength is exhausted, they must rest in their sarcophagi to regain the ability.