The Blue Scribes

Some say there was a time when Tzeentch was the greatest of all the Chaos Gods, and he ruled his brethren through his sorcerous might. In time, the other gods deposed the Great Sorcerer in a mighty battle and cast him from his perch upon the Endless Mountains.

The resulting impact shattered mighty Tzeentch into ten thousand pieces. These shards of god-stuff were flung across infinity and the might of the Great Sorcerer was forever diminished. These events form the birth of magic in many legends, for it is said that each shard of Tzeentch shifted form to become a spell or incantation. These quickly multiplied throughout mortal minds and so spread throughout eternity. Though each was too small to retain any of Tzeentch's personality, the Great Sorcerer slowly became aware of these fragments and resolved to reclaim them.

To this end, Tzeentch created two Daemons, P'tarix and Xirat'p, tasked with learning every spell in existence. Though Blue Horrors in form and surly personality, the quest required these Blue Scribes to be more self-aware than others of their kind. Ever careful of betrayal, Tzeentch bestowed this intelligence with care. P'tarix can transcribe the magical syllables of any spell to parchment, but cannot read. Xirat'p can read his brother's scribblings, but cannot understand them. Judging his work to be good, the Great Sorcerer sent his creations out into existence to complete their quest.

The Blue Scribes, sometimes known as Azure Arcanologists, Wandering Wizards, and Tzeentch's Quaestors, ride their Disc of Tzeentch through realms eternal and mortal, squabbling as they seek lost fragments of their god to bind them with parchment and ink. P'tarix scrawls frantically with a quill crafted from a Lord of Change's pinfeather. Xirat'p reads the written words to check for mistakes; in so doing unleashing the power bound within on any unfortunate enough to be nearby. The Blue Scribes' mission often draws them to battlefields, where the most destructive and powerful magics are used. If threatened, Xirat'p starts reading at random from the accumulated scrolls, trusting to the hand of fate, his master, to guide him to the correct scroll for each occasion. This can have quite spectacular and bizarre results, with a foe as likely to be struck by multicoloured lightning as he is to be drenched by his own personal thunderstorm or transmuted to solid gold.

In truth, the Blue Scribes can never complete their task, for magic has multiplied in the service of mortals. This is well for Xirat'p and P'tarix and for existence itself. Should the Blue Scribes complete their task, Tzeentch would swallow them, reuniting the lost fragments of his being and absorbing the extra power born along the way. It is doubtful that any creature, mortal or Daemon, would survive such a renewal...

Source

 * Warhammer Armies: Daemons of Chaos (8th Edition) -- pg. 41