Most of the existing Pokemon found in the Pokemon franchise have been implemented in Pokemon GO by now, but this hasn't kept the game from finding ways to keep events exciting. The upcoming Mischief Unbound event may not be introducing a new Pokemon to Pokemon GO, but it will be the debut of Hoopa's form change.
Hoopa is a dual-type Psychic/Ghost Mythical Pokémon introduced as part of Pokemon Gen 6, where it was obtainable through special events like other Mythical Pokemon. Known as the Mischief Pokemon, this spooky prankster appears in two forms: the small Hoopa Confined, its default state, and the towering Hoopa Unbound, which is unlocked with the item Prison Bottle. Although both forms are adorned with golden rings and have something of a circus aesthetic, Hoopa Unbound changes its typing to Psychic/Dark and provides a boost to the Pokemon's Attack, Special Attack, and Speed stats.
According to the official Pokemon GO site, the Mischief Unbound event will last from Friday, November 26, 2021, at 10:00 a.m. to Monday, November 29, 2021, at 8:00 p.m. local time. Players will receive two times the Transfer Candy and catch experience as event bonuses, but the event's main draw will be the Mischief Unbound Special Research, unlocked by completing the season-long Misunderstood Mischief Special Research story--or by purchasing a ticket for the Mischief Unbound Special Research in the shop for $4.99 USD. Players will need to have caught Hoopa in order to proceed with the Mischief Unbound Special Research.
Players that complete the second set of tasks in the Mischief Unbound Special Research will be able to change Hoopa's form into Hoopa Unbound. However, doing so will be a huge resource sink--going from Hoopa Confined to Hoopa Unbound will require 50 Hoopa Candy and 10,000 Stardust, while changing Hoopa Unbound back into Hoopa Confined will require 10 Hoopa Candy and 2,000 Stardust. Some of these resources can be gathered by playing through the Mischief Unbound Special Research. Also available during this event is the Hoopa T-Shirt avatar item, which will be added to the shop after the event ends for players who are unable to get it by completing the Special Research.
Raid Hour events will be held from 6:00 p.m. to 7:00 p.m on four consecutive days, each of which will feature a different set of Legendary Pokemon. November 26 will feature Articuno, Zapdos, and Moltres, November 27 will feature Heatran, November 28 will feature Regirock, Regice, and Registeel, and November 29 will feature Cobalion, Terrakion, and Virizion. The same days will also feature Rattata, Sableye, Pikachu, and Beldum respectively during Spotlight Events held from 12:00 p.m. to 1:00 p.m. local time.
Pokemon GO is available on Android and iOS.
Source: Pokemon GO