Arcahe

Also known as coffin spiders.
Elephant-sized and hard-shelled, these tailless scorpion-like beasts originated in the Eideli Empire, spreading throughout the continent. Now very rarely seen in the mid-south of the midlands due to extensive hunting, although the near-invisible northern varieties are still relatively common. Typically ambush predators; eyeless, hunting based on sound and scent. They use their large claws to grab and crush prey, and can grow scarily massive with age.
The Eideli used arcahe as pack animals and mounts. They are considered borderline divine spirits created by the goddess, and crucial to both military and day to day life. The approval of arcahe is seen as a great omen.

Biology & Abilities

Triple Planar Affinity (Fire, Darkness, Blood)

Arcahe are one of the very few examples of extraplanars that draw from multiple planes, although they can only draw from one at any given time. They have three 'abilities' linked to planes- movement of limbs, enhanced regeneration, and camouflage. Their hydraulic limbs's pressure differential mechanism is created through a link to the Plane of Fire. If pulled into an area with a strong barrier to the plane of fire, their limbs cease function completely. While this does not kill them, it does render them immobile and practically harmless. The camouflage ability is linked to darkness, utilizing Darklight to create artificial shadows and make their appearance hazier. As a result, arcahe cannot both move and shroud themselves. Those that live further north (where the link to Darkness is stronger) tend to be ambush predators, while the southern ones (where the link to Fire is stronger) tend to focus more on pursuit predation. Arcahe cannot survive in the 'true' north, as the link to Darkness overpowers all others, rendering them unable to move.
Note that in the Heartlands region, the region's Fire affinity is so strong that arcahe cannot establish a connection to Darkness sufficient to utilize their camouflage. Because arcahe will only willingly sleep while cloaking themselves, this results in them being completely unable to rest while in the area. As a result, there is no meaningful arcahe presence within the deeper Heartlands, since arcahe must leave the area to rest. During the Eideli Empire, this wasn't as much of an issue, because Wasp adherants could force the arcahe to rest regardless of cloaking (though admittedly, their campaigns never reached the true Heartlands- it may have been different at that level of ambient affinity).
Their inbuilt link to the Plane of Blood allows them to heal from massive injuries, although it leaves them vulnerable to attacks during this process. This also allows them to gain sustenance extradimensionally. Seeing as they can't camouflage during this, they often build dens to hide in beforehand.

Lifespan

Arcahe are functionally biologically immortal, and capable of reproducing without a mate. The older an arcahe gets, the less adaptable it becomes- younger arcahe's planar affinities can be influenced by the nature of their surroundings, and their regeneration is more prone to over-compensation, resulting in physical adaptations after damaging confrontations.
Embedded inside the body of every arcahe is what seems like a partially-formed human woman. It's actually very hard to discover this, seeing as the regeneration granted by Blood means that you typically have to pulp an arcahe's insides before it goes down. The body is curled around the arcahe's 'heart' (which doesn't pump blood, since they rely on Fire pressure differentials to move and not muscle contractions or typical arthropod hydraulics) within its abdomen. The few instances of this being discovered by midlanders and such generally just attribute it to the monster having swallowed a person whole shortly before (since arcahe jaws can actually open ludicrously widely when they want to). In some cases, the body is more merged with the creature, its limbs stretching out and even stretching into the arcahe's limbs. Note that arcahe are very magically-dependent in design; bring one into the mundane world and it'd simply collapse into a puddle. For example, note that they have blood, despite relying on hydraulics to actually move their limbs.

Bite

An arcahe's stingers and jaws are used to open small tears to other planes. These generally do not form a large or strong enough link to create devastating effects, but can still strike rapidly with great force. A link to Fire can create great heat, aiding the piercing of armor. A link to Blood can create rapidly spreading tumorous growths, causing excruciating pain. However, these are only surface level unless injected in massive amounts, at which point the target's insides start liquefying. Drawing from Darkness creates a slightly wispy effect from leaking darklight, creating a low-level charged-edge effect and strengthening the stinger/jaw.

Arcahe Tusk

The jaws, stingers, and the spiny parts of their pedipalps are all made of a material commonly known as arcahe tusk. Arcahe tusk sees use in a few binding rituals, and is theoretically a functional elemental dust in Epacsi enchanting.

Venom

Arcahe venom liquefies things. The effect diffuses through a target, not necessarily relying on blood flow to carry it.
The exact mechanism is conceptually comparable to forcing the flexiblity properties of stormglass on a target, and superimpositionally 'soaking in' like dilith.