On July 22nd, 2019, Niantic officially released the Team GO Rocket Invasion event. Several trainers are reporting Shadow Pokemon being available to battle across the world, featuring Team GO Rocket Grunts!
Stay tuned to this post for further details.
The event is currently being rolled out in stages to Pokemon GO accounts. If you can't use this feature yet, check back in a little while.
PokeStops that have the Team Rocket interaction will look slightly "glitched out" at a distance, changing color from black to blue and twitching periodically. Affected stops should fade from blue to black as you approach them.
PokeStops that have been taken over by Team Rocket have an "R" above them and are "greyed out".
Stops appear to be taken over by grunts for 30 minutes, though there are some conflicting reports saying this timer can decrease after being defeated by another trainer.
Players who interact with these PokeStops will have the chance to fight, and then catch, a "Shadow Pokemon". Battles are done in Master League, though the teams you'll face should be pretty easy, despite the deceptively high CP.
Upon completion, you'll receive a number of Premiere Balls relative to your performance.
Shadow Pokemon have red eyes and a ghostly purple shadow animation. They will know the charged move Frustration by default. They can be transfered, traded, and placed in gyms like normal.
Shadow Pokemon also can be purified for a small candy and stardust cost.