Initially seen on a blurred video shown by Dr. Terrence Kyne and worshipped as a divine being by the insane scientist Dr. Challus Mercer, the Hive Mind appears to be the controlling intelligence of the Necromorphs.
The Hive Mind was unwittingly created as a result of military testing of the experimental Red Marker over two centuries prior to the events of Dead Space, on the remote planet Aegis VII. When the Marker proved to be too dangerous to wield as a weapon, further experimentation with it was aborted and it was used one last time in order to suppress the Hive Mind - a massive Necromorph that was the source of the Necromorph outbreak on Aegis VII and, subsequently, the USG Ishimura.
The creature's appearance and general shape resemble a segmented worm, albeit one of titanic proportions. Its full size is never truly shown, as an unknown portion of the creature is hidden inside a crater. Its visible portion, however, consists of a discernable head and thorax. Due to its size and appearance, it is safe to assume that, while not the same size, it has the same body dimensions of the Nexus.
The head's most prominent feature is a massive circular mouth, consisting of two sets of jaws, each with their own rows of teeth. The Hive Mind is able to spit acidic projectiles from its central mouth. Around the mouth there are five yellow-glowing sacs that strongly resemble eyes. The thorax features several branched appendages on the back and a frontal cavity which shelters five more yellow sacks, which are apparently vital organs.
The Hive Mind is capable of utilizing its immense tentacles for manipulation and offense. It uses three of them during the fight with Isaac; however it almost certainly has more than three. The tentacles grow from some part of the creature that is well below its thorax area.
While initially thought to be a sentience and the 'command center' for the Necromorph hive-mind, it is now known that it was merely a drone. The true controlling source were the Brethren Moons, or more specifically, the Tau Volantis Moon.
After reawakening, the Hive Mind becomes aggressive and attacks Isaac and the escaping Kendra as they make it into the shuttle bay. After killing Kendra, it curls its tentacles around Isaac, forming an inescapable enclosure, initiating the final fight. In the following fight, Isaac managed to defeat the Hive Mind by destroying what appeared to be its vital organs and it fell off the edge of the colony. What happened to the Hive Mind afterwards is unknown, but it can safely be assumed that the Hive Mind was ultimately killed, either after its battle with Isaac, when the portion of the planet initially removed by USG Ishimura fell on top of the colony and the Hive Mind, or when Aegis VII was destroyed.