I don't see how it would be OP. Since it's immaterial and will thus be invulnerable when played, you can't buff it beyond one Nightfall/Eclipse. That means it will return 2.5/3.5 at most. This would only be a good investment if -all- the attacking creatures suffered the damage, but I don't see that happening with how Gravity Pull works. A lot of creatures will OHKO this, meaning the damage will only go back to one creature. The upped version wouldn't even kill an upped Shrieker. Not worth playing.
Also, you need to clean up the card text. "If Pain Mirror takes damage from a creature, inflict half that damage back at the creature." works, but it's a bit long. "Return" is a bit ambiguous, which is why I wanted to move away from it.
Oh, and did you ask for permission to use the art? If so, you need to credit the artist on the card itself. Also, you're missing a
in the table card text.