From Halopedia, the Halo wiki
Sigma Chi (in Greek letters, ΣΧ) is one of the largest and oldest college Greek-letter secret and social fraternities in the world.
Sigma Chi was founded on June 28, 1855 at Miami University in Oxford, Ohio when members split from Delta Kappa Epsilon. Sigma Chi's purpose is to promote the concepts of friendship, justice, and learning; its mission statement is to develop value-based leaders committed to the betterment of character, campus and community. Sigma Chi's vision statement is "to become the preeminent collegiate leadership development organization—aligned, focused and living our core values".
The fraternity's official colors are blue and old gold, and its badge is a white cross with emblems on each of its arms: crossed keys on the upper arm, an eagle's head on the right arm, seven gold stars and a pair of clasped hands on the lower arm, and a scroll on the left arm. In the center of the cross, on a black background, are the gold symbols for the Greek letters Sigma (Σ) and Chi (Χ). The left and right arms are connected to the upper arm by gold chains.
In 2552, an ONI agent disguised himself as a mugger as a way of testing potential new ONI recruit, Rani Sobeck. However, Rani Soback found out about the ONI agent because of his Sigma Chi tattoo, which, along with expensive clothing, indicated he was not a simple mugger.