Retro Pikachu Event
The Retro Pikachu Event begun on the 01/Oct/2014 and ended on 15/Oct/2014. It was then recycled as an event from 5/Oct/2016 to 20/Oct/2016.
Task 1 - Light Balls
When clicking the Pikachu Icon on the userbar, a page will load up with a Pikachu asking you to collect 20 Light Balls. You can obtain them by interacting. 1 Light Ball can be obtained after 150-300 interactions.
After obtaining all 20 
Light Balls, a blue "Pika!" button will appear beneath the Light Ball count. By clicking it, you will unlock a new special Rumble Area called the Yellow Forest.
Yellow forest
Once completing task 1 of the Retro Pikachu event, a new Rumble area will be unlocked called "Yellow Forest".
The Yellow Forest can only be explored by Pichu, Satichu, Pikachu, Satochu, Raichu, Raitoshi, and Retro Pikachu.
When sending out a Pokémon to the Yellow Forest, it will have the opportunity to find a Retro Egg, which will hatch out to be a Retro Pikachu. The Pokémon may also find Tiny Mushrooms, and berries. Pikachu and Pichu can also be recruited while your Pokémon is out on the mission. There is a higher chance to find a Shiny Pichu/Pikachu via rumbling in the Yellow Forest. After the 2016 Event Distribution, Pokémon had a chance to recruit Spiky-eared Pichu.
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Light Balls are no longer be obtainable, but you will still be able to send your Pokemon to the Yellow Forest Rumble Area (if unlocked). Pichu's and Pikachu's will still be obtainable, and so will Retro Pikachu, but at a lower chance.