From Halopedia, the Halo wiki
The piece is a fast paced piano and electric guitar piece meant to accompany action sequences of similar pace, it later incorporates strings playing a variation of a segment of the Halo Theme. It uses the same rhythm as Rock Anthem for Saving the World did in Halo: Combat Evolved.
In The Package from Halo Legends, a rearrangement of Unyielding plays during the scene where John-117, Kelly-087 and Frederic-104 are fighting their way through the Covenant ship, and finishes just after John and Kelly are separated. The piece was not included in the Halo Legends: Original Soundtrack.