An umbral agent is shadow that was never alive.
Malstaris is crowded with shadows that used to be people. When a mortal dies, its shadow sinks here, and what arrives is a husk called a shade. An umbral agent is not one of those. It was never anyone. No soul stands behind it, no dead person's strand is knotted inside it, and there is no spirit it once held and lost. It is native shadow-substance, born of Malstaris and belonging to it, the way the Devourers and Mind Fiends and Voids of the deep wastes are. Almost everything else in the shadow-realm is the leavings of a life that ended on the other side of death. Umbral agents are the plane's own, and that is the whole difference between them and the shades they move among.
The shadow-realm's enforcers
They are called agents because they act for something other than themselves. The Malstaric kingdoms of the dead are kept in order, and the shade-crowds do not keep themselves. The Parliament of Shadows and the Shadow Lords who hold the regions of Malstaris do not herd shadows with their own hands, so umbral agents do it for them. They work the borders of the shadow-kingdoms and the thick of the crowds, turning back the dead that drift where they should not, and unmaking the ones that will not turn. A shade caught badly out of place is simply taken apart.
None of this is ordered in detail. An umbral agent takes no oath and keeps no post. It hunts in the direction the shadow-realm's powers want it hunted, and the arrangement is loose enough that those powers rarely have to look at it twice.
A border between two of the shadow-kingdoms, where the milling dead thin to a trickle. Something moves along the line that is darker than the dark around it. It has no face and no settled edge, and it folds and unfolds as it goes. The shades nearest it drift away without appearing to decide to, the way water parts around a stone. It makes no sound. It has nowhere it is going, and it never stops going there.
First through the rift
What an umbral agent does in Malstaris, it carries into the living world the moment it gets the chance. During a Shadowrift, when the wall between Malstaris and the Material Plane goes thin, umbral agents are the first things across. The larger hunters of the shadow-realm come behind them. The agents arrive first because driving and breaking strays is the work they already know, and a flooded surface is suddenly full of strays.
On the living side they have nothing to enforce, so they only hunt. They treat the people of the surface the way they treat a shadow loose in the wrong region, as something to be run down and unmade. They belong to none of it, and they stay for none of it. When the rift closes and the dark that let them across drains back down, they go with it.