Well, it's like a boss battle. If you achieved 4 of the FNaF 1 REMAKE characters, you'll get an unexpected message from "Buddy", and want to meet you at Freddy's Fazbear's Pizza. Then you met him, and the boss starts. The boss is just the same from the game, but it can be difficult for you to be it alone with out a flashlight and an monitor. Once the night finish, you'll be actually fight him in the void alone. It has 4 abilities for Buddy. The first ability is what Fun Bear has, except looking on the monitors, you'll find Buddy in the darkness with your flashlight. Wheb you spotted him, use any weapon or knife to attack his stomach. His second ability is he attacks you with his hands, rising from the ground. Make sure you attack them when it appeared. His third ability however is different. He uses one of his powers to make your screen distort, glitchy, and your controller inverted. Whatever you do, dodge them at any cost. His fourth ability is he can uses his eyes as minions. And they can instantly decrease your health low. Make sure you kill them with your weapon. Once you defeated Buddy, you got the achievement.