Gravity falls

An open book with intricate drawings flips rapidly, transitioning to a bright flash, then reveals a man in a lab coat and a floating triangular character in a dimly lit room with a fireplace, followed by two small figures running down a dark staircase, then a group of silhouetted characters facing a central figure in a red-lit chamber, and finally the triangular character sitting beside a large man in a yellow armchair with blue flames in the background.