When Desire Hath Conceived

Sin and Death from John Milton's Paradise Lost, specifically Book II Line 650. Be warned, though, their tale gets rather... miserable.

When Satan first conspired against God, he keeled over from a severe pain in his head -- his evil thoughts had manifested into a female being who proceeded to smash her way out of her father's head! (particularly, out of the left or sinister side of his noggin) The other soon-to-be-fallen angels were horrified by this and named the newborn Sin, but Satan fell MADLY in love...

Sin was thrown out of Heaven along with the rest of the traitors, though she left with the sole key out of Hell and a "growing burden." Soon she gave birth to Death, who made an utter mess of her body on the way out. Sin was absolutely terrified by her nightmarish child and tried to flee, but Death caught up to her and went Oedipus. This unholy union spawned a brood of yelping hellhounds who dive back into the womb whenever they're frightened, and comfort themselves by gnawing on their poor mother's innards.

Eventually Satan would meet his progeny while making an escape from Hell. After nearly coming to blows with Death and having to have Sin's sad tale told to him, Satan (rather easily) convinces his offspring to open the gates, and the terrible trio wreak havoc on the world.