Raikou and the Timed Incubator to GO Pass and GO Pass Deluxe in Pokémon GO starting March 3, complete Pass Tasks to rank up your GO Pass to earn an encounter with Raikou and more rewards

Starting March 3, Pokemon GO Trainers with a GO Pass or GO Pass Deluxe can take on new timed challenges to earn an encounter with Raikou. Completing Pass Tasks helps you rank up your GO Pass, unlocking Raikou and other rewards along the way. The update adds a fresh layer to regular GO Pass gameplay, giving players extra goals and incentives for participating in raids and field activities. If you have been waiting to catch this Legendary, the March rollout is a perfect chance to power up your Electric-type lineup.

Source: Official Pokémon GO blog