His Mother's Adversary

Back during the war with Homeworld, the Crystal Gems had a enemy, beyond the general threat of the diamonds. Homeworld had one gem that was way out of control; Beyond the Diamond's oversight even. This gem was Iolite, a prototype for a new war gem; however, she was imprisoned on earth for her excessively brutal fighting style and disregard of orders. But now, she's back with one directive. To crush what's left of the rebellion she sought to destroy 5,300 years ago.

Cover credit to "ben-ben" on Deviantart.