Dirge of Cerberus: Final Fantasy VII
Voice Actor: N/A
Unlike other weapons created by the planet (e.g. Diamond Weapon), Omega Weapon isn't designed to try and protect the planet from danger. Instead, its job is to transfer the Lifesteam to a new planet should there be a threat so big that the life of the planet is itself in danger.
After absorbing the Lifesteam, the current planet will wither and die, but Omega's job is then to find a new planet for the Lifestream to inhabit. This will enable the cycle of the Lifestream to continue.
Summoned Under False Pretences
After learning about Omega Weapon from Lucrecia Crescent's research, Hojo hatches a plan to try and take over Omega Weapon so that he can use its awesome power for his own means.
His death allows him to take over the mind of Weiss and using his influence, he sets his scheme in motion. The plan is to sacrifice as many pure souls as possible so that Omega thinks the planet is under legitimate threat.
Once Omega had been summoned, Hojo would use Weiss' perfect body to fuse with it, but this didn't go to plan as Nero tainted the body and Hojo's consciousness was lost.
Chaos vs Omega
Due to Omega Weapon's colossal size, nobody can seem to stop it and even after the Mako Reactors are destroyed it still seems to grow in size and power. Lucrecia theorised that Chaos was the antithesis of Omega, and thus, Vincent is the only one who can defeat Omega - which he duly does.