Crystyls were extra-planar crystalline entities in the I.C.E. Age, typically 10 to 20 feet individually, with the ability to exist in multiple places or even planes of existence simultaneously. While they were extremely intelligent and tremendous sources of information, they lacked the interest or motivations typical of intelligent lifeforms. It was only possible to communicate with them with mental spells, and they would not otherwise be inclined to speak with anything. When threatened they would club the victim or stab it with their shards, and would shift itself elsewhere or another plane entirely.
They were thought to be distantly related in some way to earth elementals. Though they could communicate with mental magic, they had no ability to cast such spells themselves. Crystyls would ordinarily move very slowly if at all, and had no need to eat or sleep. Their homeworld, if any, was unknown. Those in The Broken Lands were presumably made artificially, and may be reincarnated by the dome. It is impossible to control the crystyls with magic. However, this paradox is also present with other entities in the Broken Lands, and the implication is that Uthex used them for his extra-planar research.
Behind the Scenes
The crystal forest on the jagged plain of The Broken Lands is implicitly a large cluster of crystyl entities. These were in the same category of "other standard" extra-planar creatures in the source material used for the other works of Uthex Kathiasas. Similar to the magru, hoard, and giant fog beetles, crystyls were notorious for infesting random planes or simultaneously existing in multiple worlds. The crystyls could not be summoned or controlled. However, they enjoyed adding gemstones to themselves as they were each unique in appearance, and would willingly trade information for gems.
The following is what the crystyls look like on the jagged plain. There seems to be no way to interact with it, 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 crystyls, and perhaps even keep them bound to this plane of existence, which may explain why they have become a whole crystal forest.
[The Broken Lands, Jagged Plain] The dense tangle of wildly growing crystals to the east presents a formidable barrier. While travel through the rocks and boulders of the jagged plains is difficult, trying to make your way through the sharp outcroppings of the crystal forest would be deadly. Obvious paths: south, southwest, west, northwest >look crystals Translucent white crystals grow to great heights here, sprouting sharp spears in all directions that form an impassable barrier.
If the crystyls correspond to something from "The Dream-Quest of Unknown Kadath", it might be the ending rant of the Other God Nyarlathotep, who speaks of the "great dome" in the crystallized memories and dreams of the quest seeker's youth. This is the scene that is alluded to by Purgatory and below The Graveyard. It is interesting because Iruaric is actually a telepathic language. The possibility of the crystyls storing information, or even consciousness across time and space, is reminiscent of the Lord of Essaence "speaking crystal" Bandur attempted to steal from Nomikos. Their basic omnipresence along with the hoard omniscience, and the multi-planar coexistences of the analogous locations, implies Yog-Sothoth who resembled iridescent globes and corresponds to Kadaena.
The crystal forest of crystyl entities may correspond to the "gigantic neighbouring prism-clusters" 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 Hoard might be cross-referenced with the mud footprints of something trans-dimensional in that story.