Recommended: Level up your health to the max and take about 1-3 whole pizzas
1) Finding Black Rabbit:
Once you have unlocked access to the inside of the castle, go to the top left room with a long table and you will see a wall on the left that is glitching. Walk through it until you find a chest with a key named ‘???’. Then go back to the entrance of the North Forest where you will find the bird bath and ‘use the key’ on the bird bath. You will now be transported to a darker version of the world and tasked with finding 6 parts of the Black Rabbit, 1 part in the West, South, East North forests, inside the Castle, and in the central hub. Once all parts are placed back at the Bird Bath, you will need to find a well within the South Forest past a boulder(dig under with shovel). Once you interact with it you will be transported to recreations of the FNaF 3 mini-games. Simply keep walking left through the walls until you find a glitch beginning a Corrupt Bonnie Fight.
2) Once in the fight, DO NOT ATTACK BONNIE, but instead keep blocking and using you healing items. Black Rabbit will then tell a story and end by thanking you for listening to them. The achievement should appear soon after being sent to the main menu.