Divine Gate

The Divine Gate is a barrier that prevents the gods of Exandria, both good and evil, from physically crossing into the Prime Material Plane. The more powerful a supernatural creature is, the harder it is for the creature to pass through the Divine Gate. It is permeable to lesser supernatural creatures and to mortals who have the magic to travel between planes.

The Gate was made by the Prime Deities after the Calamity to better protect mortals from the influence of the Betrayer Gods. The creation of the Gate marked the event known as the Divergence.

Structure of the Divine Gate
The Divine Gate resembles a burning tapestry suspended in the blackness of the Astral Sea. It is filled with many portals. It is an esoteric, non-Euclidean structure that is a barrier between the Prime Material Plane and everything else. The Feywild and the Shadowfell border closest to the Prime Material Plane, but there is still a fine veil between them based on the Divine Gate.

The Planes of Existence
Members of Vox Machina and the Mighty Nein were able to travel between the Material Plane and several other Planes of Existence without noticing a passage through the Gate:
 * Inner Planes: The Elemental Planes of Fire, Earth, Air, and Water
 * Several of these instances involved moving through gateways or rifts, rather than using a spell like Plane Shift.  But even when using a spell, there was no mention of the Gate when Keyleth carried the party to or from the Elemental Plane of Fire. When Jester used Plane Shift to return from the Elemental Plane of Fire to Exandria, there was no mention of anything resembling flight through the Divine Gate; instead, she and her group were enveloped in the green cloth of Artagan's cloak and suddenly appeared at her destination.
 * Overlapping the Material Plane
 * The Feywild
 * The Shadowfell
 * The Ethereal Plane

In the deepest reaches of the Underdark, Vox Machina also touched the edge of a thin membrane, or veil, separating the Material Plane from elements of the Far Realm.

The Mighty Nein did not notice the Divine Gate when passing through the Immensus Gate to the Astral Plane but did notice it when using Plane Shift to return.

Using Keyleth's Plane Shift spell, Vox Machina encountered and passed through the Gate during their visits to each of the following:
 * Outer Planes:
 * The Nine Hells
 * Elysium, using the tuning fork gifted by Sarenrae
 * Pandemonium
 * Unknown Plane
 * The Endless Athenaeum, Ioun's secret realm
 * Travel to the plane was via Sprigg. This Plane Shift spell took noticeably longer than Vox Machina's other planar travel.
 * The Gate was again visible while returning.

The Astral Sea
This space between the planes is the matrix in which the Gate and the planes are suspended.

Strength and significance
A unanimous effort of the Prime Deities or possibly an unsealed Tharizdun working alone could break the Divine Gate, which would lead to another Armageddon on the mortal realm.

The ascension of Vecna in presented a new problem for Exandria. Vecna did not need to cross the Gate to enter Exandria; he simply achieved godhood on the mortal side of it. This marked the first time since the Gate's construction that a deity had free rein in the mortal world.

Furthermore, when the gods last clashed during the Calamity, they were all fighting on the same plane. During Vecna's ascension, the Prime Deities could not physically join in the fight, as they were still held back by the Gate. So, not only was Vecna a god on the wrong side of the Gate; he was also a god unchecked by any other gods.