Halo: Ultimatum

HPA is one of the many secretive projects created by the Office of Naval Intelligence. Utilising canidates originaly planned for the SPARTAN-III Project, they carry out assasinations and other covert ops under total secresy and with no remorse. They are killers to the bone, with no limitations on how to conduct their buisness. However, when one of those missions intercepts an operation conducted by the UNSCDF-MC SPARTAN-141, sparks fly as the  lead agent  and the enigmatic SPARTAN-II clash.