The wife of Dominus Tenebrous, she was seen as a being of fear and dread. An enemy of Caelus and harbinger of destruction. Yet she came to them as a friend, someone who wished to help them when the other Gods would not. She fanned their flames of bitterness, claiming the Gods could have given them wings but simply chose not to. Denying what they wished out of spite and to keep them under their control.
Tempestia offered them the wings they so craved, if they just pledged their souls and essences to her forevermore. The mountain Benelim, eager for the chance to fly, accepted her terms. Thus the Benelim were altered by the magic of Tempestia and the first Harpies were brought into being. Rejoicing the harpies soared into the skies, relishing in their new found freedom.
But as they flew, they spotted a trading caravan of Benelim and a strange urge overtook them. A bloodlust, a desire to spread chaos! Like wailing sprites they descended on the merchants, slaughtering them in an orgy of carnage. The harpies realized in horror what they had just done. Tempestia laughed at their plight, and revealed that as they are bound to her, they serve her desires now, for Tempestia is a being of chaos.
Some of the harpies valiantly refused to play along, claiming they would beg for help from Terra or hide themselves away. Tempestia cared nothing for their words and punished them in retaliation. The dissenters turned to dust. Tempestia warned the rest that they must always spread chaos and discord where ever they go. If they wait too long or try to flake out of their service, they would turn to dust as well. Despair and madness filled the hearts of the remaining harpies and they accepted that this was their life now.
Over the centuries, such madness warped into a fanatical devotion. The harpies love their patron deity like few others. Generations of harpies sang the praises of their chaotic master. For Tempestia's part, she was not without gratitude. For their centuries of service, the eldest of the harpies found themselves gifted extra sets of wings and even the ability to cast storm magic, in imitation of the Storm Goddess herself. As a result, never has their bond been stronger.
A Life of Discord
Harpies are usually solitary creatures. Sometimes they congregate in loose tribes and groups. Harpy 'society' is matriarchal and indeed male harpies are less common and typically smaller. This is because Tempestia herself has a dismissive view of men and that reflects in her servants. These alliances never last long, for Tempestia loathes the idea of such organization.
Harpies infest the world, they go after everyone and anyone. Like predators they seek out the vulnerable, the weak and the sick. Many public events or showings have been undermined by a cluster of harpies showing up to cause havoc. Many of them join up with Maelim warbands, for they know where ever a warband goes, so does chaos. The Maelim usually don't mind, as long as they save their bloodlust for their enemies and not them.
Not all harpies are so insidious. While they must spread discord, some prefer finding ways that don't involve spilling blood. Stealing food from a feast, destroying a noblemen's garden, etc. It is theorized some remnant of sanity endures in some way, a piece of who they used to be, or could have been. A fragment of empathy and mercy. They know their bond is unbreakable, but believe they can have their cake and eat it too.
Tempestia tolerates this for the most part, since it's a decent way to mix things up and its the chaos and discord that matters the most. Yet eventually, inevitably, even these more gentle harpies will succumb to the bloodlust like the rest. Such is the eternal servitude to the Goddess of Chaos.