The Planar Seal

The brainchild of Ovi Varn, the Planar Seal was a dimensional shield that covered Erdos during the Age of the Varn. Its express purpose was to prevent deities and other extraplanar entities from influencing the people of Erdos. The trigger that caused the Varn to make this decision, was the increased interference of deities and clerics in the Varn’s ongoing agenda to make the Lands a better place for all.
The Seal extended to just beyond the planet’s exosphere. Within the seal, all magic that dealt with conjuring or summoning spirits and objects from other planes and universes, as well as all forms of teleportation and planar travel, were rendered ineffective. In addition, Erdos’ connections to its two physical counterparts in the Feywild and Shadowfell were severed, as was contact with the ethereal plane. As a result, the crossings to those realms stopped existing. All of the random portals that could lead to Sigil, were also prevented from manifesting. Basically, if it had anything to do with another plane of existence—including demiplanes, but not extradimensional spaces—it stopped working.
Contrary to popular belief, users of divine magic such as ancient clerics and paladins did not lose access to their magic. However, no new divine magicians could be created, as the ritual of investiture necessary to grand a creature access to magical power required (and still requires) contact with deities. Consequently, divine magicians gradually became a thing of the past.
The Planar Seal was projected by the Citadel of the Varn; in fact, the infrastructure of the colossal edifice, was designed specifically for that purpose.
The Planar Seal necessitated the invention of new magical techniques to replace certain magical effects that were no longer possible, chief among them being teleportation. Ordinarily, such magic relied on the astral plane to create connections from portal to portal. To solve this, the Varn created the magical principle of demispaces; a cross between a demiplane and an extradimensional space. They then used their Citadel once again to project a massive demispace called the Terminal Nexus, which would replace the astral plane as the transitive medium of teleportation magic.
This required magicians of the time to invent new iterations of the teleportation circle and teleport spells, as well as the creation of a new spell called linked portal. The Terminal Nexus could not extend beyond Erdos, and so those spells could not be used to reach other worlds. Similarly, there was no method possible to bypass the Planar Seal and cast planeshift or similar spells.
The Planar Seal made Erdos impossible to reach from agents of other planes, while Erdos’ magis absorbing properties as a magicant world, rendered the planet extremely difficult to reach via spelljammers. This put Erdos on the unique position of having zero influence from any outside force.

End of the Seal

At the conclusion of the War of the Varn, Ovi lifted the Planar Seal and locked the entire Citadel of the Varn within a demispace localized to Akualinor, before departing for planes unknown.
The lifting of the seal allowed the deities and their religions to reemerge once again. With the Citadel gone, so was the Terminal Nexus. It would take nearly two centuries for Shadokhan, the Archmage Supreme, to reinvent the original teleportation spells that rely on the astral plane.