Shadow Pokémon

Shadow Pokémon are mysterious Pokémon that Team GO Rocket Grunts and Leaders leave behind after they lose the battle. They are distinguished from normal Pokémon by a burning purple aura, glowing red eyes and often more menacing expression. Shadow Pokémon can be cured by purifying them to become Purified Pokémon with Stardust and Candy. All Shadow Pokémon that are captured will know the attack Frustration. This feature is only available to Trainers who are Level 8 and above.

Obtaining Shadow Pokémon
Team GO Rocket is invading PokéStops all around the world. These PokéStops have a different appearance than regular PokéStops in the Map View. These PokéStops twitch and appear discolored. As a Trainer approaches an invaded PokéStop, it becomes black and the Team GO Rocket logo will appear over it. If a Trainer interacts with the PokéStop (items are still given) or the Grunt, a battle will ensue in order to defend the invaded PokéStop. Winning Team GO Rocket Battles offers a chance to catch Shadow Pokémon and increases the rank of the Hero medal. The PokéStop will be invaded for 30 minutes at a time.

Team GO Rocket Battles work similarly to Trainer Battles. Trainers will need to select a party of three Pokémon to defeat the opponent's Shadow Pokémon. After winning a Team GO Rocket Battle, 500 Stardust will be rewarded and there will be a chance to catch one of the Shadow Pokémon they have left behind. If the battle is lost, a rematch will be offered. Team GO Rocket does not use protect shields.

Warning: Some players have reported that having an incomplete default Ultra League team would prevent usage of any Pokémon above 1,500 CP in this battle.

Enemy Shadow Pokémon fought in Team GO Rocket Battles have greatly increased CP above normal limits, similar to Raid Bosses, albeit not as much; roughly 3 times the regular maximum of the Pokémon in the wild. It appears their strength is offensive-biased, meaning they can dish out quite a lot of damage but does not take hits as well as their CP suggests.

Similar to Raid Battles, Premier Balls, will be used to capture left behind Shadow Pokémon. At least three Premier Balls are given after a victory, and the number of extra Premier Balls received (beyond the base number of two) is based on how many Pokémon survive the battle, the rank of the "Purify" medal, and the "Hero" medal.

The Shadow Pokémon available for capture is usually the first one battled, but there is also chance of catching the second one.

When attempting to capture Shadow Pokémon, they attack far more often than normal, making them especially hard to catch. A Nanab Berry can counter this. Also, while Shadow Pokémon often have a deep orange ring (even for common, low-level Pokémon like Rattata) it seems to be misleading, as the capture rate is not as low as the capture circle implies.

Purification
After capturing a Shadow Pokémon, there will be an option to purify the Pokémon with a combination of Stardust and Candy (ranges from 1000 Stardust and 1 Candy, to at least 5000 Stardust and 5 Candies). Evolving a Shadow Pokémon does not affect the cost, which appears to be based on Buddy Pokémon walking distance: 1 Candy for each km required to walk to gain 1 Candy, and 1000x the amount in Stardust. For example, you must walk a Snorlax for 5 km to obtain 1 candy; therefore, it costs 5 Candies and 5000 Stardust to purify a Shadow Snorlax.

Purifying a Shadow Pokémon have the following effects: Purifying a Shadow Pokémon increases the rank of the Purifier medal. The Pokémon's dark aura will also change to a bright, luminous aura. Purified Pokémon require a Special Trade while Shadow Pokémon cannot be traded.
 * CP is boosted to level 25. If the Shadow Pokémon has been powered up past level 25, it would stay at the higher level (not recommended as powering them up costs 3x as much).
 * Amount of Stardust and Candy required to Power Up, Evolve, or learn a new attack reduced.
 * The Charged Attack, Frustration, will be replaced with the exclusive Charged Attack called Return.
 * Individual Values of the Pokémon will all increase by 2.

Improvement costs
The following table shows the costs for powering-up, evolving, or adding a second attack to Pokémon. The Stardust cost reduction stacks multiplicatively with Lucky Pokémon (-50% cost); a Pokémon that is both Lucky and Purified costs 45% the normal amount of Stardust to power up.

List of Shadow Pokémon
Shadow Pokémon were made available in different waves. See the linked list for information about purification cost and release date.

Trivia

 * Shadow Pokémon was introduced in July, 2019 with the launch of Team GO Rocket Invasion.
 * However, the feature was suspended one day later. It was found that the adjustments include reduced catch rate, increased attack and dodge rate and Shadow Pokémon are banned from trading.
 * Shadow Pokémon was released again on July, 2019. After a few days, the candy and stardust purification costs were adjusted to current values (it was previously not based on walking distances, and evolved Shadow Pokémon of the same family can have different costs, e.g. Wartortle costed more than Squirtle). In most cases, it is a reduction; exceptions include Squirtle, Charmander and Bulbasaur, which used to cost 2 Candies and 2000 Stardust but now costs 3 and 3000.
 * According the official blog:
 * Shadow Pokémon look unusual and behave strangely because Team GO Rocket tried to make them stronger through unnatural means.
 * Purification help Shadow Pokémon return to a more normal state, but Purified Pokémon can also become stronger than their normal counterparts due to the gratitude they feel toward the Trainer who saved them.
 * The catch circle of Shadow Pokémon is always dark red, but the Pokémon appear to have normal catch rate.
 * The number of each Purified Pokémon a Trainer has purified is tracked in the Pokédex.
 * Shadow Pokémon all share a unique background not based on Types. It is dark purple and a silhouette of the Pokémon can be faintly seen.
 * The background of Shadow Pokémon completely override that of Lucky Pokémon.
 * The animation for purification is similar to Evolution. Similar to the latter, purification can be done even if the Pokémon has fainted, and would revive them to full or near full health.
 * The number of Premier Balls you obtain is increased by 1 upon your tenth defeat of a Rocket Grunt, even though technically you don't gain the badge until after the catch.
 * It has been observed that purified Turtwig does not offer a reduced cost in evolution, but the cost is still reduced for unlocking a second charged attack. This difference may be only during Turtwig's Community Day on September 15, 2019.
 * Shadow Pokémon cannot learn Community Day Charged exclusive moves. They will keep Frustration after evolved. Trainers must purify first in order to learn exclusive move, which replaces Return.
 * Unlocking the second Charged Attack has no effect on this.