What is the general scenario of the campaigns in HeroesV?

The general scenario of the campaigns can be considered a struggle between races for dominance and/or the defense of their own lands during another invasion of demons from the underground prison of Sheol (read: Hell). First, humans fight off the demons with varying success; then, a messenger from Sheol weaves his intrigues; next, Necromancers try to seize power in the human empire; after that, a defector from Sheol reincarnated as a dark elf helps the humans while fighting off rivals for power in the kingdom of the dark pointy-ears; following this, elves enter battle against the Necromancers who have taken over the empire; and the final campaign is about a mage reclaiming his lands from the Necromancers, who, uniting with elves, the remnants of humans, and dark elves, descends into Sheol, saves the queen of the empire from the indecent claims of the local boss, and that's it, we all won :D