You could begin by setting the scene. Describe the environment or the mood to draw the reader in. Also, introduce a character in an interesting way to pique curiosity.
Once upon a time in the spooky world of FNAF, Foxy was known for his sudden and terrifying appearances. There was a night guard who heard strange creaking sounds coming from Pirate Cove. As he shone his flashlight towards it, he saw Foxy's glowing eyes in the darkness. Before he could react, Foxy dashed out at lightning speed, his sharp teeth bared. The guard barely escaped that night, haunted by the image of Foxy forever.
Once upon a time in the FNAF Gacha Life world, there was a girl character who worked at Freddy Fazbear's Pizza. She fell in love with a male animatronic. They would secretly meet at the backstage. The girl would bring the animatronic little gifts like new gears to fix his parts. Their love was against all the odds as it was a very strange combination, but they didn't care and just enjoyed their time together.
The intro of Pulp Fiction is a captivating sequence that sets the tone for the entire movie. It features some unique characters and dialogues that draw you in right from the start.
Well, in a Harry Potter and FNAF crossover fanfiction, the magic of Harry Potter could interact with the horror of FNAF in unique ways. The Death Eaters could use the animatronics as some sort of dark weapon. And Harry and his friends would have to use their spells and magical skills to stop them. Also, the Hogwarts houses could have different reactions to the animatronics. Gryffindors might be brave enough to confront them directly, while Hufflepuffs might try to befriend them in a way.