Across eons and unimaginable distances, an endless hoard of slithering and gibbering nightmares have assailed the mindscape of every world since the dawn of sentience. However, not every race native to the strange Far Lands is inimicable or predatory to sentient beings. Some of agents of hopeful and helpful, if surreal, dreams, rather than nightmares. Thus were the Quori warrior, monks, and philosophers who left the realm Dreams to defend the mortals of Kor during the Mindgate War. Powerful in the realm of imagination and thought, they could do little about those Aberrant creatures who had taken fleshly forms. Thus did they invent the Kalashtari Contract, inspired (almost certainly) by the symbiosis first forged on a distant world almost a millennia before the Mindgate War.
Kalashtar are formed via an agreement between a sentient mortal and a Quori. A symbiotic relationship is formed, allowing the two minds to merge. For the duration of the contract, the two are one being. Different Contracts can be of very different tenors. Some are intimate, like lovers. Others are businesslike affairs, and still others leave the Quori completely in charge while the host mind slumbers. Each Contract is entirely voluntary. By their nature, the Quori can neither mislead the host, nor force the symbiosis. Sometimes, the host wishes to drift in endless pleasant dreams fashioned for it by the Quori. Others seek active communication, a fully shared experience. In most cases, the Contract is lifelong, but sometimes the pair separate sooner. The Contract cannot be formed through nefarious means, but it can be extended by them. While rare, evil Kalashtar have formed over time. No being is immune to corruption.
In every case, the purpose of the Kalashtar is to safeguard the Mindscape by protecting the sentient beings who create it through their very existence. When their Contract dissolves, the Quori will continue their quest, working to protect Dreams and Thoughts until they find a new host with whom to forge a new Contract. Some tire of the Contracts, and live on only in Dreams. Others get so entangled with their host that they perish with them, drawn into whatever afterlife the two have earned. Others cannot sustain themselves at all, dissipating into the etheric landscape of the Mind until only echoes remain. Some have never rested, tirelessly forging new Contracts with the death or departure of each host, ever vigilant for any Aberrant enemies.