From PokéHeroes Wiki
Shadow Pokémon outbreak
A new big feature has been added: Shadow Pokémon are now available in a radar mini game! After reaching Trainerlevel 20, you can get the new "Shadow Radar" at the Item Shop. With this item, you can chain and catch rare shadow Pokémon for your PokéDex. The available species vary every day - and after gaining some hunting experience, you will find rarer species out there. Currently, only Pokémon from the Kanto-region are available, but more will be added very soon.
Read the news post to get all the details on the new updates: