Mona Lisa

The UNSC Mona Lisa was a prisoner ship that was in service into the late years of the Human-Covenant War. Being a prison ship, it was in reality a converted freighter, meant to transport ore and prisoners to and from penal colonies. As it was, it was mostly unarmed, as prisoners' ships usually were, though mysteriously without an escort to prevent escaping convicts.

Installation 04
When it arrived at the wreckage of Installation 04 in Soell System weeks after the Halo Event, its true purpose was put into action. Surveying the wreckage and finding Sangheili survivors, the ship then took them as prisoners. One of particular interest to the ONI operatives on board was one suffering from a strange infection. ONI planned to study the infection, either looking for a cure or way to control it. However the Elites and human test subjects broke quarantine and infected the ship, beginning the process of building a Proto-Gravemind to attempt FTL travel without needing access to the bridge, which was locked off.

However, when the UNSC Red Horse arrived at Installation 04, a squad of Marines among her crew was sent to investigate it, at first presuming it derelict. They split up, looking for survivors, but were quickly alerted to the horrors of the ship. Among its survivors were "Henry", an Elite who teamed with a prisoner named Rimmer, and John Smith, an ONI operative responsible for the experiments on board. The ship is then ordered to be destroyed by Rebecca, the Red Horse AI, by a Shiva-class Nuclear Missile. The squad was slowly whittled down as the action continued, until they reach the escape pods. Only four members of the squad survived (Lopez, MacGraw, Clarence, and Benti), along with Henry and Rimmer. While upon reaching the escape pods, Clarence shots Rimmer and it is revealed that he was an ONI operative sent to "clean up the mess". The remaining survivors are caught in a standoff pointing guns at one another as the Flood is approaching their position and during this pause, MacGraw takes off in one of the escape pods. A wounded Benti jumps on Clarence and takes them both out into the hallway where they are overtaken by the Flood. Sergeant Lopez, the leader of the squad, and "Henry" fought over the last remaining escape pod, presumably to one or both of their deaths. The ship was subsequently destroyed.