The Planes

Layers of Reality

Iroa does not have the vast panoply of planes that characterize the "default" D&D setting. Instead, it has just a few. This is unlikely to come up at low levels, but becomes more relevant at higher levels, especially if anyone in the party chooses to go after spells or items that interact with the planes.
However, a spell is not always required. The planes touch Iroa, and bleed into it and each other at their edges. It is entirely possible to take a very wrong turn - or a very right turn depending on one's goals - and wander right out of the mortal world into one of these more fundamental dimensions.
Most characters will only have the most basic understanding of the Astra, Crixos, and Nod, and won't have much conception of Yeven at all.

Astra

The heavens, the canvas of Fate, the realm of Empyrean spirits, generally accepted to exist "above" the mortal realm. Reality is said to emerge from the Astra as water from a spring, its paths and currents determined by the fates written in the stars, "trickling down" across the "tiers" of existence and becoming more "solid" and defined the further it courses. The Astra in turn is said to be shaped by mortal belief, the ambrosia of the Gods, who move among the heavens and weave their blessings, curses and schemes here to shape the future of the world.
In traditional D&D cosmology terms, Iroa does not have an Astral Plane, the Upper Planes, a Positive Energy Plane, or a Plane of Time. The Astra is all and none of these. Spells and powers which rely on the aforementioned planes instead channel or touch the Astra.
Characters trained in Arcane have a basic understanding of the Astra, those trained in Religion know more about its inhabitants, and those with both have a better grasp of how it affects the world.

Crixos

The underworld, the paths of the dead, the realm of Chthonic spirits, generally accepted to exist "beneath" the mortal realm. Crixos is more fixed and codified than mortal reality itself - pitiless, immutable and immovable save by the most extraordinary of efforts. Crixos's river of souls is fed by the constant death of all living things, but in turn waters the roots of the world and binds the world to its history so it is not unmoored from past and future. Crixos is said to be not only the destination of the dead but also the source of life, the font from which new souls are born, and thus ironically for such an unchanging realm may ultimately be the source of the power to change and redefine the world and reality itself.
In traditional D&D cosmology terms, Iroa does not have a Fugue Plane, the Lower Planes, or a Negative Energy Plane. Crixos is all and none of these. Spells and powers which rely on the aforementioned planes instead channel or touch Crixos.
Characters trained in Arcane have a basic understanding of Crixos, those trained in Religion know more about its inhabitants, and those with both have a better grasp of how it affects the world.

Nod

A term for "the Dream," "the veil," "the borders of the world," and so forth. It is the plane that separates but is simultaneously coterminous with mortal reality, the Astra and Crixos. It is the collective subconscious made real, the Dreamtime, an echo of reality. Nod is defined by stories, concepts and precepts rather than by physical law. Dreams, souls, faith energy, and the threads of Fate are believed to flow within Nod, which is said to be how the souls of the dead reach Crixos, how the stars of the Astra shape mortal destiny, and how extraordinary dreams can touch the Dead below and the Gods above. In its role as all of these things, Nod is said to be the wellspring of heka, the power of magic.
In traditional D&D cosmology terms, Iroa does not have an Ethereal plane, a Feywild, a Shadowfell, or a Plane of Mirrors. Nod is all and none of these. Spells and powers which rely on the aforementioned planes instead channel or touch Nod.
Characters trained in Arcane have a basic understanding of Nod, those trained in Nature know more about its inhabitants, and those with both have a better grasp of how it affects the world.

Type

Dimensional plane