To coincide with Pokémon Festa 2018, a new Pokémon GO Week is taking place this month for fans in South Korea. From September 17 to September 25, rare spawns of certain Pokémon will appear more frequently in the wild. Specifically, from September 21 to September 23, rare Pokémon such as Feebas and Trapinch will appear in Seoul while select letters of Unown and Tropius will appear around the Lotte department store and Lotte World Mall. This event is open in admission and free for anyone to take part in. In addition, while supplies last, participants will be able to receive an exclusive Pokémon GO-themed strap. The official announcement from the development team at Niantic is as follows:
Pokémon GO Week in Korea!
Pokémon Korea, Inc., will host the Pokémon Festa from Friday, September 14, to Wednesday, September 26. For the duration of this event, Trainers all throughout South Korea will find more Pokémon appearing across the country in Pokémon GO!
And from Friday, September 21, to Sunday, September 23, even more surprises will arrive in Seoul, South Korea. Pokémon such as Feebas and Trapinch will be in the area for this special event. Plus, Trainers at Lotte World Mall will be able to find Pokémon not normally seen in Korea, such as the Grass- and Flying-type Pokémon Tropius and the Psychic-type Pokémon Unown!
Please visit Lotte World Mall to encounter these Pokémon!
—Pokémon GO team
Source: Official Pokémon GO blog