Retro Pikachu Event
The Retro Pikachu Event begun on the 01st of October, 2014. It was not announced before hand, but a Pikachu Icon appeared on everyone's userbar, showing a new event begun.
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 Pokemon to the Yellow Forest, it will have the opportunity to find a Retro Egg, which will hatch out to be a Retro Pikachu. The Pokemon may also find Tiny Mushrooms, and berries. Pikachu's and Pichu's can also be recruited while your Pokemon is out on the mission. There is a higher chance to find a Shiny Picu/Pikachu via rumbling in the Yellow Forest.