"Make us whole..."
Convergence is an event that occurs in the Dead Space universe. It is the final process of a Marker and the affected lifeforms of a given planet (or planets) in which the Marker draws to it all organic matter that has been infected with the Necromorph contagion.
When a sufficient number of organic lifeforms have been turned into Necromorphs, the Marker that controls them signals them to gather around it, fusing together in order to create a Brethren Moon, the final stage of the Necromorph life cycle. Necrotic organic tissue gathers and compiles in a planet's stratosphere to form the Brethren Moon's body and the gravimetric force generated by the Marker will also draw in non-organic debris over a large radius as the Moon grows larger. When the Brother Moon is fully formed, the creators of the Marker must also die and their flesh added to the Moon's mass. The sentient moon will then proceed to seek out and consume organic life across the galaxy.
The Necromorph scourge has spread across the galaxy in this manner for an unknown amount of time and each Convergence Event has resulted in the birth of a new Brother Moon at the cost of the extinction of entire species. By the 26th century, virtually all of the Milky Way galaxy is dead with Earth possibly being the only planet left with any life upon it. Humanity has also fallen victim to the Markers, however, as a Marker was discovered on Earth in 2214. This "Black Marker" has been speculated of manipulating humanity's evolution in order to create a species intelligent enough to create more Markers for the purpose of triggering another Convergence.
The planet of Tau Volantis underwent Convergence centuries past, but the process was halted when the planet's inhabitants used a great machine to freeze their entire world and prevent the birth of a new Brethren Moon. In the year 2514, the Convergence was briefly restarted by Isaac Clarke, but he was able to resist the Marker's influence long enough to stop the Tau Volantis Moon and destroy it before it became too powerful. While that Moon was destroyed, the rest of the Brethren were signalled to Earth and Isaac was unable to stop their arrival.