Whats my favourite bit of the Storm of Magic book? Simple, its the bottom half of page 33 which details the spell The Seven Secret Sigils of Summoning. With this spell it allows you to summon a single unit of 300, 150 or 75 points. Anything you like as long as its no monster or unique creature.
It allows you to take some of the famous heros already out there, but more importantly it allows you to create new ones of your own, complete with a backstory of how they come to be summoned to the fight. From my own stable I present:
Kha'loth - The Champion of War
As the storm of magic billows and the sorcerers and mages of the world wallow in power so the warriors of the world find their role diminished. To Khorne, god of war, such things are intolerable and so into these storms the blood god sends a mighty champion, Kha'loth, to restore the balance and bring humility back to wizard-kind. Astride a bellowing Juggernaught and shielded from all sorcery it is said even his blood is an anathema to magic users.
Kha'loth is a Chaos Lord with the Mark of Khorne, Riding a Juggernaught and equipped with a shield and Acid Ichor Chaos gift*, He comes in at a perfect 300pts exactly and certainly has the power to upset a few casters.
*Presently the rules state all the units options are permitted, excluding magic items. Chaos gifts have yet to be clarified.