During chapter 7 Bus stop. From the bus stop head to the right into the woods take the right path and follow the path up past where your sister was eating the deer to path leading to a noodle shop for spirits talk to the tall spirit behind the counter she will give you a bowl of ramen for yourself and a bowl to feed a spirit now head back to the fork in the road and take the left path you will see a purple spirit with antlers talk to him them use the bowl of ramen on him to unlock your achievement.
**You need to do this achievement in order to obtain the mushroom needed to complete the Mushrooms make you bigger! achievement.**