The story of the White Tempests starts with the birth of the Old Ones.
Being a powerfully psychic race they made an imprint on the then calm Warp; also possessing a cold, focused intellect these psychic emissions fused into one powerful entity. This being became know as Bahamut, Fist King of the Empyrean.
Fast Forward->
Following the “War in Heaven” the lack of Old Ones and the emergence of the three new Chaos Gods left Bahamut weakened, until he came to a deal with Tzeentch: in exchange for power and support Bahamut took up the role of Guardian of the Immaterium.
In more recent times the Daemon King has busied himself with combating the rising threat of the Necrons, studying the Tau’s lack of warp profile and more prominently seeking out and investigating any and all ruins of Old Ones facilities and research outposts.
To aid him in his endeavours Bahamut has gathered a war host of sorts comprised of:
Alastor, his Daemon Lieutenant from the war against the C'tan, now trapped in a suit of Power Armour;
Prestidigitation Squad, a group of Tactical Dreadnought Armoured mercenaries;
Abra Squad, a recent attempt to reproduce Daemon possessed armour like Alastor;
Kadabra Squad, six half-crazed not to mention possessed Noise Marines thrill-seekers;
Alakazam Squad, Thousand Sons led by a self-exiled loyalist marine plagued with a parasitic daemon;
And finally two Daemon Engines, one piloting a stolen, experimental Tau Battlesuit, the other a fully fledged Defiler named Spindleshanks.
And that is the much abridged history of the White Tempests.