Lumbering figures of living gray matter from Affril, Plane of Knowledge.

Across the room, translucent gray figures walk across a purplestone archway from another world into the king’s chambers, while gigantic brown bats circle the air behind them. The gray figures touch everything within reach, dissolving objects as if pouring acid from their fingertips. - Eye of Ulos (RPA-ADV4)

They can absorb visions and memories into their forms by touching objects and people, dissolving them in the process. The memories of objects/lifeforms touched by Grayfolk in other planes are returned to the gray sea in Affril, Plane of Knowledge, for Klynoks to study.

On Ulos, the Grayfolk can reduce objects to nothing by touching them, then absorbing and displaying the memories of those things in their forms;

As each object melts away, visions flicker through the bodies of these strange invaders. One of them dissolves a lamp, and its body is briefly filled with a vision of Taron frantically writing letters by the lamp’s light. Another creature dissolves a pillow, and its chest lights up with a vision of Queen Amalia, Taron’s late wife, lying awake at night.

The strange invaders’ bodies are lit up with visions from the guards’ lives — scenes of their families, their pastimes, and their memories of the Dragul Invasion. - Eye of Ulos (RPA-ADV4)

The Grayfolk can assume a larger, combined form if threatened;

The Grayfolk begin to pool together. A larger, unified monstrosity now presses forward with a dozen corrosive arms stretching out to encircle you. Your nerve begins to falter. - Eye of Ulos (RPA-ADV4)

It also appears that Grayfolk must have some connection to their home plane Affril, Plane of Knowledge, else they are reduced to stagnant pools of muck;

As the rift seals upward, the creatures melt into puddles on the floor.

Within their remains, you now see movement, as if the life force of the creatures persisted even though they can no longer hold their form. - Eye of Ulos (RPA-ADV4)

Grayfolk appear at Sabek Palace to take King Taron to their plane.

A Grayfolk slips through the rift of another gladiator who bears the mark of the Starlit Door at the Colosseum of Ashes:

• Warn her of the Grayfolk's presence and she kills it, then moves onto you.
  • You finish her quickly on the ash.
  • She disables you with her bow and spares you for saving her life.
• Stay silent and it dissolves her, then moves onto you.
  • You reduce it to a puddle of goo with your weapons.
  • It has you on the ropes, but turns to muck when the rift behind it closes.

A Grayfolk fights under the command of a Starlit Door cultist in Affril, Plane of Knowledge at the final battle of Brizoor Waal.