Frustration

Frustration is a Normal-type charged attack. All Shadow Pokémon has it upon capture, but is replaced with Return upon purification.

Frustration is retained by Shadow Pokémon if evolved. Charged TM does not work on it. Shadow Pokémon cannot learn Community Day exclusive moves.

Removing Frustration
During certain events, Charged TM could be used to change a Shadow Pokémon's Frustration to other Charged Attacks that that Pokémon could learn normally. Shadow Pokémon and their evolution can learn exclusive attacks in future events as long as they forget Frustration.
 * Global take-over mini-event - March, 2020, 2 p.m. to 5 p.m. (local time)
 * Battle-themed event - July, 2020, 11 a.m. to 2 p.m. (local time)
 * Wooper Watch - December, 2020, 4 p.m. to December 2020, 4 p.m. (PST)
 * Special Raid Challenge - February, 2021, 11 a.m. to 2 p.m. (local time)