Who is the main Forerunner antagonist in Halo 4?

The Didact is the main Forerunner antagonist in Halo 4.

He is awakened from imprisonment on the Forerunner world Requiem after the UNSC ship Forward Unto Dawn is pulled into the planet's gravity well. The Didact regards humanity as a dangerous species and seeks to use the Composer to convert humans into digital entities called Prometheans.

The character is a central figure in Forerunner-era lore, which was expanded through Greg Bear's Forerunner Saga novels. Halo 4 presents him as an ancient military leader rather than a Covenant commander or Flood creature.

A common mix-up is the Librarian, who is also a major Forerunner character but opposes the Didact and supports humanity. The Ur-Didact refers to the same original character in later expanded fiction, while Halo 4 generally identifies him simply as the Didact.