Once the capital of the Covenant and the jewel of their star-spanning empire, High Charity is now a shattered collection of wreckage sealed off and buried on the Ark's surface. Only a small fraction of its grandeur remains, and even that is distorted and tarnished by the Flood trapped within its halls after the Gravemind was defeated by the Master Chief.
Decimated-but not eradicated-the parasite has marshalled what remains of its strength in the years since. This diseased cluster could have been cleansed if the installation's monitor was still active, but the Ark's fallback defense protocols can only contain the Flood inside High Charity with armored walls and patrolling Sentinels, a half-measure that allows the parasite to fester and plan within the ruins.