Now you have to live the rest of your life knowing you're going to die someday and disappear forever.
Necromorphs are mutated and reanimated corpses, reshaped into horrific new forms by a recombinant extraterrestrial infection. The resulting creatures are extremely aggressive and will attack any uninfected organism on sight. The Necromorphs are the main antagonists of the Dead Space franchise.
The sole purpose of almost all Necromorphs was to acquire more bodies to convert and spread the infection. They are believed by some to be the heralds of humanity's ascension, but on a more practical level are the extremely dangerous result of exposure to the enigmatic devices known as the Markers.
The Necromorphs are controlled via the Markers by a group of entities known as the Brethren Moons which are massive Necromorphs created by killing and infecting every living thing on a planet. The Necromorphs' motivations are to create a mass of necrotic flesh to be collected by the Markers during a Convergence Event with the intent to create another Brethren Moon.
It can be seen during the final battle.