The natures of the various dark powers of the Earth is one of the least explored elements of Earthsea's cosmology. In particular, the episode at the Court of the Terrenon in Osskil is probably the most mysterious part of A Wizard of Earthsea. However, it does appear that Ged believes that all the shadowy entities that he encounters are somehow related. This includes the Terrenon Stone, the dark powers of the Tombs, and the shadow creature named Ged.
The black, winged things that are called up to hunt after Ged and Serret take on ancient forms.
The creatures returned to the attack: botched beasts, belonging to ages before bird or dragon or man, long since forgotten by the daylight but recalled by the ancient, malign, unforgetting power of the Stone.
The nature of the things summoned up to prevent Ged's escape from the Court of the Terrenon is never discussed in any further detail. However, it appears that they are extensions of the evil entity imprisoned in the Stone itself, not separate creatures in the Stone's thrall. They are things that it has created out of its own essence, and so they are bound to the land where the Stone holds sway.
The creatures of the Stone circled a while croaking, and one by one beat back ponderously inland over the moors. The Old Powers will not cross over the sea, being bound each to an isle, a certain place, cave or stone or welling spring.
There seems to be a distinct similarity between the evil essence contained in the Stone of Terrenon and the evil powers that rule over the labyrinth at Atuan. Both are ineluctably tied to the place where their power is focused and cannot stretch their influence over the sea to anywhere else.
"What have they ever given you, Tenar?"
"Nothing," she whispered.
"They have nothing to give. They have no power of making. All their power is to darken and destroy. They cannot leave this place; they are this place; and it should be left to them.
However, rather than summoning fell creatures to hunt Tenar and Ged, the Masters of the Tombs cause an earthquake, trying to trap and kill the pair, although Ged manages to hold back the quake until he and Tenar are away and out of danger.
Finally, Sparrowhawk also tells Tenar that the shadow monster Ged that he fought was somehow related to these other dark powers, but that is not further elaborated.