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 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.

Vox Machina's ability to cross the Gate indicates that it is permeable to mortals, though not to deities.

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 divides the Planes of Existence into two sides: the mortal and the divine.

The Mortal Side
Any planes that Vox Machina have visited without passing through the Gate can be presumed to be on the mortal side. This includes any planes that connect directly onto the Material Plane.
 * The Material Plane itself
 * The Elemental Planes of Fire, Earth, Air, and Water
 * The Feywild
 * The Shadowfell
 * The Far Realm

The Divine Side
Presumably, any plane inhabited by a deity would fall on this side of the Gate, but Vox Machina have only visited three (using Keyleth's Plane Shift spell). They encountered, and passed through, the Gate during their visits to each of the following:
 * The Nine Hells.
 * Elysium, using the tuning fork gifted by Sarenrae
 * Ioun's secret home plane, using Sprigg
 * The Plane Shift spell took longer than usual in this case, possibly because this hidden plane was more distant than the planes they had previously visited.
 * Pandemonium.

The Astral Sea
This space between the planes does not seem to fall on one side or the other, but is the matrix in which the Gate is suspended.

Significance
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 had free reign in the mortal world.

Furthermore, when the gods last clashed during the Calamity, they were all fighting on the same plane. During Vecna's ascention, 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.