High Charity
From Halopedia, the Halo wiki
Summary
High Charity is the name of the mobile capital city of The Covenant. It is a large bulb-shaped structure, which is actually a massive starship. It is capable of slipstream jumps, like any other vessel. The city houses the majority of structures sacred to the Covenant, such as the chamber of the Council, the Mausoleum of the Arbiter, and the sanctum of the Prophets. The power source is in fact a Forerunner ship which had been modified to power the entire city.
Before the fall of Reach and the destruction of the Pillar of Autumn at Alpha Halo I-04, the UNSC planned to have that ship transport Spartans to capture a Covenant ship, in order to locate High Charity. There, they would capture the High Prophets, and force an end to the war. During the events of Halo: First Strike, however, Master Chief gave up on the mission despite being in possession of Ascendant Justice due to the persuasion of his fellow survivors of Alpha Halo's destruction. During Halo 2, the Chief (who was alone teleported to that same Covenant homeworld by Gravemind) apparently had forgotten about his original mission already, because instead of using the perfect opportunity given to him and going to capture the High Prophets, he decides to kill them instead, in which, according to Dr. Halsey's profile on Covenant society, would escalate the war. Master Chief receives aid from Marine POWS, not his Spartans this time. Due to this, a potential novelisation of Halo 2 that addressed the plotholes created by the game could be very different, because prior characterisation of the Chief in the earlier novels has portrayed him as one to be very careful in his actions.
During the course of Halo 2, The Flood take advantage of the chaos surrounding Master Chief's infilitration, using the situation to infest the city. Eventually, the the Forerunner ship at High Charity's core takes off, with Master Chief inside. Cortana stays behind. The city is abandoned and assimilated by the Flood, drifting in the middle of a space battle. Whether it will be reinhabited or destroyed in the future (Halo 3) is unknown.
Images
http://img170.exs.cx/img170/9260/highcharity9eh.jpg
