AT&T is buying Time Warner in a $85.4 billion deal that could bring sweeping changes to the information and entertainment economies. (Video: Jhaan Elker/The Washington Post)

AT&T said Saturday it had agreed to a blockbuster $85.4 billion acquisition of Time Warner, a move that would turn America’s most storied telecom company into one of the most prominent TV, film and video-game producers in the world.

The deal casts a spotlight on a defining movement for the giants of modern tech: Their accelerating conquest of media in an increasingly unbundled world.