By serving the Master, we will survive the glorious cataclysm to come. Through his eyes and as his hands, we are made mighty and will transcend the boundaries of mortal flesh.
Many lack the strength to overcome the shackles of feeble morality and embrace the voice in the shadow. These many will stand against you, call you heretic and monster and fool, and demand you bow to their false gods.
Others still shall join your cause, and be incapable of satisfying the expectations of His Verminous Majesty. They are a weakness that must be purged, lest they ruin the Master's plan and drag down his loyal servants with them.
Take their souls from them as price for their hatred, so they may be made useful. Before the sight of the Master and in His temple, invoke the ritual that shall burn away their impurities and create them anew.
As the crucible holds molten metal to be purified, the black soul gem acts as the reservoir for their spirits in Oblivion.
Those who are purified in such a fashion shall not — indeed, cannot — taste death so long as their gems endure.