Hoard were extra-planar collective entities in the I.C.E. Age, resembling seas or humanoid mud forms, while actually being colonies of mutually self-aware single celled organisms. Hoard were able to sense all of the other hoard colonies across planes of existence, and when threatened would all know who and where to go if possible for retribution. In the humanoid form they were twelve feet tall, coincidentally the same height as the avatars of deities, and when fighting could split themselves in half to increase their numbers. The colony would only die if its parts were too dispersed.
Hoard had a strange ability to sense dimensional warps they could travel through, and would wander across many planes of existence with no apparent homeworld. These had sorcerous and mentalist powers such as pain infliction, and physical contact with them would cause a mind jolt kind of effect. While they did not have native plane shifting powers as did the Nycorac and Crystyls, they did have innate phase powers which is relevant to the doom of Shadow Valley as it was originally near the portal. It was impossible to summon hoard, and they were immune to slashing and puncturing. However, those of The Broken Lands may have been artificial, owing to the work of Uthex Kathiasas. Twelve feet is the height of the crystal dome as well as the statue of Morgu (Marlu) in the Dark Shrine.
Behind the Scenes
The grey mud sea on the jagged plain of The Broken Lands is implicitly made up of Hoard colonies. These were "other standard" extra-planar entities known to exist on many planes, similar to the source material behind other creatures such as the magru, dark vorteces, and crystyls. These all tended to have unknown homeworlds, the inability to be summoned or only accidentally, and being known for finding their way onto other planes of existence. Hoard, crystyls, and traag were only implicitly present in the game, whereas Uthex gave the others different names mostly in the dead language Iruaric.
The following is what hoard look like on the jagged plain. There seems to be no way to interact with them, and telepathy does not work in The Broken Lands. It is impossible to "reach out with your senses" outside the workshop, because "something seems to be blocking them." When you are too close to the crystal dome it interferes with transportation magic, sometimes Spirit Guide (130) stops working. These factors interfere with the possibility of communicating with the hoard, and perhaps even keep them bound to this plane of existence, which may explain why they have become a whole sea of mud.
[The Broken Lands, Jagged Plain] A huge sea of boiling mud stretches out to the south and east. Steam rises off the churning mass, choking the air with a dense, malodorous fog. Obvious paths: north, southwest, west, northwest >look mud The mud is predominantly a grey-brown color, but there are patches of green and orange mud here and there. A thick, grey steam rises off the surface of the mud.
The boiling mud is one of the environmental hazards on the Jagged Plain, suddenly raining down on top of you in a way that would not make sense for mud. It will also hit you in rooms away from the mud, which if not a bug would refer to the Hoard phasing ability. These hazards may have been turned off. This hazard and the random meteor effect were active on at least the test server in late 2017.
A torrent of thick sludge suddenly comes raining down on you! After a brief moment of surprise, you realize that the stuff is boiling hot! You are injured by the boiling hot sludge! ... 5 points of damage! Minor burns to right leg. That hurts a bit.
If the hoard correspond to something from "The Dream-Quest of Unknown Kadath", it might be when the quest seeker "sought a forest pool and cleansed himself of the mud of nether earth." The forest in this context would be the "crystal forest" of crystyls to their north, with the two seemingly avoiding each other. It is interesting because Iruaric is actually a telepathic language. While the boiling sea forms a dense fog, the mist circles the crystal dome and thins to the north, possibly alluding to being hunted by shoggoths in "At the Mountains of Madness" where the haze is mistaken for volcanism. This idea of the shoggoth mist actually arising from the "blasphemous, horror-fostering abyss" appears also in Shadow Valley. As the dark vorteces may be an unnatural hybridization of the Nycorac and Blacar, the magru may be of Absorbers and Hoard (shoggoths are globs of cells.) The sea of mud would be sleeping or dormant magru, much as the streams and ponds with color variations in Shadow Valley.
These originate in globules of "the colour" from "The Colour Out of Space", which is a cross-reference with the shoggoths below the fallen red-litten city of Yoth, whose ruins were below the blue-litten Tsath. The bubbling grey mud, in the analog of the well of Shadow Valley filled with human and small animal skeletons, was described as: "The ooze and slime at the bottom seemed inexplicably porous and bubbling, and a man who descended on hand-holds with a long pole found that he could sink the wooden shaft to any depth in the mud of the floor without meeting any solid obstruction." Their basic omniscience along with the crystyl omnipresence, and the multi-planar coexistences of the analogous locations, implies Yog-Sothoth who resembled iridescent globes and corresponds to Kadaena.
It is also possible that the Hoard are being cross-referenced with the mud footprints from something trans-dimensional in "The Dreams in the Witch-House", which is a Lovecraft story possibly relevant to Castle Anwyn that is similarly themed to "Through the Gates of the Silver Key". Shortly after the story mentions Azathoth and Nyarlathotep, the climactic figures from the "Dream-Quest of Unknown Kadath", it describes a dreamed otherworld scene identical to the jagged plain: "Gilman was half-involuntarily moving about in the twilight abysses with the bubble-mass and the small polyhedron floating ahead, when he noticed the peculiarly regular angles formed by the edges of some gigantic neighbouring prism-clusters. In another second he was out of the abyss and standing tremulously on a rocky hillside bathed in intense, diffused green light. He was barefooted and in his night-clothes, and when he tried to walk discovered that he could scarcely lift his feet. A swirling vapour hid everything but the immediate sloping terrain from sight, and he shrank from the thought of the sounds that might surge out of that vapour." The prism-clusters would correspond to the crystal forest of crystyl entities.