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.

For a list of Shadow Pokémon, see List of Shadow Pokémon for purification cost or List of Shadow Pokémon by release date.

Obtaining Shadow Pokémon
In order to obtain Shadow Pokémon, Trainer has to defeat any member of Team GO Rocket (including Grunts, Leaders or Giovanni) and participate in a Rescue Shadow Pokémon challenge that is similar to the Bonus Challenge in Raid Battles.

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

Shadow Pokémon that are being captured after battle with Leaders can be shiny.


 * See Team GO Rocket for more information on the Rescuing Shadow Pokémon.

Purification
After capturing a Shadow Pokémon, there will be an option to purify the Pokémon with a combination of Stardust and Candy. Evolving a Shadow Pokémon does not affect the cost, which appears to be based on Buddy Pokémon walking distance.

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.
 * 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 multipliers 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.

Shadow Bonus
Starting on March, 2020, Shadow Pokémon received bonus to make them stronger. Shadow bonus applies to all Gym, Raid and Trainer Battle. The attack bonus can be stacked with weather bonus.
 * The attack of Shadow Pokémon is multiplied by 1.2.
 * The defense of Shadow Pokémon is multiplied by 0.833.

The cost of Power Up and the second Charged Attack requires less Stardust and Candy. The multiplier went from 3 to 1.2.

Trivia

 * Shadow Pokémon was introduced in July, 2019 with the launch of PokéStops takeover feature.
 * 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.
 * 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.
 * 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.
 * 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.