The Inner Planes
The eighteen realms beneath the world's fabric: elemental, polar, and the mixtures between them.
Where the Outer Planes are ordered by conscience, the Inner Planes are ordered by substance. They are not realms a soul is judged into but the stuff the world is made from: four elements, two poles of living and unliving energy, and the twelve mixtures that occur where those meet. Eighteen seats in all. Fifteen have a god sitting in them. Three do not.
Two authorities that never met agree on that arrangement. The pantheon names each Ethereal god by the plane they hold, and has done so for as long as the faiths have been written down. Against that, at the Twisted Grove in S3E38, the Elder, a Peren ghost bound in the keeps, described the same architecture from the inside, naming a Plane of Radiance no living scholar had cause to name and a Plane of Ash beside it.
Reaching Them
The Inner Planes are nearer than the Outer, and more dangerous for it. The veil thins on its own in places called the motes, one to each element, and the sodden forest of Gaba Alam is the Water mote by every measure taken there. The Mól did not wait for thin places. They cut crystals at the planar boundaries and bound what came through, and their keeps each straddle a pair of diametrically opposed planes. The Muharib were set to guard the sites.
Souls travel the same roads at the end. They are meant to join the Inner Planes at death, drawn on the Soular Winds, and to become over centuries a part of the plane that claimed them. Clan Saytar tried to force that passage early. The curses are what came of it.