Final answer:
The first fantastic event in 'House Taken Over' is when an unseen presence forces the siblings out of a section of their house.
Step-by-step explanation:
The first fantastic event that occurs in the story 'House Taken Over' is when the siblings are subtly and inexplicably driven out from a section of their house by an unseen and unknown presence. They hear a strange sound, which forces them to abandon part of their home and steadily looms as a paranormal occurrence. This introduction of a supernatural element marks the beginning of a series of incidents that lead to the climax of the story, where the fantastic intertwines with the characters' reality.