Mortal Realm

The Spheres exist on the Mortal Realm. Time passes consistently, gravity is a law of existence, and mortals are born, live, and die. While Spirits, denizens of other planes, and other incorporeal entities can exist for short times on the Mortal Realm, without something to tether them they will eventually be forced off the plane into the Astral Sea and the Maelstrom, or dissipate into oblivion.

Fonts

While the border between the Mortal Realm and the other planes is durable, it is not impassable. Fonts that connect to the various Elemental or Energy Planes exist and allow the passage of energies and entities to pass through to the Moral Realm.

Portals

Since portals to other planes have to pass through the Astral Sea they tend to be unstable or the Maelstrom infuses them, damaging all that pass through. As such beings that pass through portals are effected by the Maelstrom unless extreme defenses are built to counteract those effects. Currently only the Pathkeepers have that knowledge.