This was a real highlight of the Shaw Brothers catalogue for me when they were released in the 00's. Sadly the melodrama missed the mark on this rewatch, which made the plot's absurd contrivances harder to look past.
Cheng Pei-Pei is of course a delight, and the main reason to watch the film. Her range here is something to behold - her expression effortlessly flipping from psycho killer to lovesick puppy to fatalistic regret at the drop of a hat. Chang Yi is hopelessly outmatched (not one of Shaw Brothers' most expressive actors).
Considering when it was made, The Thundering Sword is a decent effort, but I can't rank it amongst the Shaw Brothers essentials anymore.
I guess this is an adaptation of Heaven Sword and Dragon Sabre, though a completely different part to the films with that name and Kung Fu Cult Master.