The Silence

The Voice describes an unknown, but powerful entity that forced the Doctor's TARDIS to explode, and thus created the cracks that would erase the Universe. (DW: The Pandorica Opens/The Big Bang)

Although the entity behind the voice was never visible or named, it was heard to be speaking before the TARDIS exploded, saying: "Silence will fall." (DW: The Pandorica Opens)