Low Charity is the name given to the Flood Hive that houses the Gravemind. The Hive itself is huge, almost the size of a continent, and contains billions of Flood organisms. The Hive was created from the remnants of the Covenant holy city of High Charity, in which theProphet of Truth lived along with all the other Prophets that were still alive after their homeworld mysteriously disappeared in a ball of fire (Exploded, basically). Since the Flood infestation, the majority of the Prophets have been killed or infected, leaving only around 1,000.
The Hive itself consists of many chambers, some still bearing resemblance to the High Charity rooms. However, the superstructure of the city has been dramatically changed, with some rooms being unrecognisable over the layers of Flood biomass that have grown there. Not only that, but damage to the city has caused some rooms to become unstable, with some being tilted at strange angles and others being turned completely upside down to suit the will of the Gravemind.
The Hive appeared through a portal over the Ark, a gigantic Forerunner structure that controls the Halo Array, (and damaged the Shitmaster's Go-Kart in the process) and the slipspace rupture tore the already critically damaged station apart, leaving only the Flood biomass keeping the superstructure together. If this wasn't bad enough, the Hive then crashed on The Ark, and the Majority of the Hive was destroyed when Installation 04B was fired. However, Gravemind actually survived this attack and formed a peaceful co-existence with the rest of the Universe after being introduced to the League.
Since then, Low Charity was bore little resemblence to its original state. Now it looks more like a ball of Flood biomass about the size of a small moon, with only a few vestiges of its Covenant-constructed origins remaining. The Hive is composed entirely of the Graveminds body, with other Flood Forms seeking refuge within the massive structure. Gravemind now uses Low Charity as a sort of fairground attraction. He lands at random fairs throughout League Territory and offers his services to the fair. The attraction itself consists of a massive walkthrough Low Charity, which is twisted by Gravespeare into a demented funhouse. The attraction proves very popular with fairgoers, guarenteed to scare, and so many fair owners are delighted when Low Charity shows up on their grounds, even paying the docking fee for it.