Sentient shadows cast by the light deities present inside unrefined starstone. Their history with the starstone is unclear, but centuries of confinement under the light's watchful eyes appears to have driven the tain to an animalistic madness. Or they may have been that way from the start. It's hard to say.
The tain is a hivemind that manifests in proportion to the amount of starlight in the area, essentially the antithesis of energy. Desperate for substance outside themselves, they absorb anything they find appealing into their ranks to try and make themselves stronger. It appears they do this purely out of primal instinct; creatures this vicious couldn't possibly organize enough to have ulterior motives.