An Unown's Hidden Power, despite the mystique of millennia and many a theory, is nothing more than language. Alone an Unown is futile, a stray letter drifting in the breeze, but it is able to join with other pokemon in a myriad of different ways, giving each a different power. They are most powerful combined with other Unown: a team of one Unown I, one K, and two L's can produce an attack which slays most foes in a single hit. In large circles the size of an alphabet or two, they form word after word, shifting with a fascinating array of tactics requiring the creativity of a Scrabble champion to use effectively. And most importantly, they can communicate over distances not through saying their name or charades, but as easily as humans use neon billboards and the spoken word.
Indeed, it is primarily from Unown that human language developed. The power words of an Unown's attacks and their combinations with existing pokemon provide the building blocks of our language. Consider the term "ghastly", developed to describe Unown H combined with Gastly, an eerie mix which make regular Gastly seem harmless. And although words also came from the need to describe new concepts, and many others originated from the names spoken by other pokemon, one must only recall the Unown techniques of "THE" for targeting and "AND" for summoning teammates to remember how strongly the Unown grip the roots of our speech. After all, every form of writing known to man has developed from either pictographs or stylized representation of the Unown.
Some have sought to tap into the Unown's power in order to conquer the world. These people are called writers, and they have already done so a thousand times over.