The Psulron are bogeyman even to the Ylph and the Illithid. So little is known about them and no one understands what they want. They were the Gardeners in their Sphere and came to dominate it after the Architects faded. Even surpassed them in many ways. But unlike the Impeyrion, they stopped their dominion at the edge of their Sphere.
The Ylph were also Gardeners, but after they left their Sphere, they created a material empire, taking over multiple Spheres. The urge to dominate amongst the powerful is easy to understand. Restraint is not. By comparison, it is almost frightening.
The Psulron are hunting Glitches outsise their Sphere.