Originating as a corrupted version of the ancient custodians known as the Precursors, the Flood is a highly virulent parasitic species that thrives by consuming sentient lifeforms with sufficient biomass and cognitive capacity.
Through the process of infection, you can assimilate other organisms, seizing control of their bodies and nervous systems to transform them into specialized forms within your collective. As you infect sentient beings, you absorb their memories and intelligence, thereby increasing the overall cognitive capabilities of the Flood as a whole. As a Gravemind, you are the shared consciousness and ruler of the Flood, capable of telepathic communication. Controlling individual instances of your hive becomes more challenging, but exerting influence over larger groups or the entirety of your assimilated hive becomes easier. To further expand your reach and processing power, you can create Key-Minds, which act as extensions of your influence.
Another benefit of being Gravemind is that you transcend the limitations of mere biological existence. In alignment with the Flood's strategy of infiltrating all available channels of information, you possess the ability to target non-biological intelligences and data networks with advanced versions of the logic plague—a form of information-based infection unique to the Flood. This allows you to infect artificial intelligences, harness any technology you have assimilated, and often surpass the original creators in terms of understanding and utilization due to the vast knowledge and intellect you have absorbed.
With the assimilation of entire planets and the emergence of planet-engulfing Gravemind forms, the collective intelligence and processing power of the Flood grant you the ability to manipulate neural physics. Through the coordination of multiple planetary Key-Minds, you can even manipulate space-time itself. One example of this manipulation is the Flood's connection to neural physics, which caused the Forerunners (an advanced ancient civilization in the Halo universe) to perceive the surrounding space-time as increasingly hostile on an abstract level.