Crimson Alone

Crimson Alone is an achievement in Halo 4. It is awarded for completing a Spartan Ops chapter solo on Legendary difficulty. The achievement is worth 20 Gamerpoints. It is represented by an eagle with a human skull on top of it.

Trivia
The achievement may be a reference to Fireteam Crimson who actually participate in several of the missions for Spartan Ops.