Metatron's Cube
Metatron's Cube is a sigil which opens a portal to Heaven, although it was not explicitly referred to as "Metatron's Cube" by the angels.
After Metatron expelled all the angels and locked the Gates of Heaven, he created a piece of spellwork that allowed him to come and go from Heaven to Earth.[1] The spell simply required a specific sigil to be drawn. Once done, it would create a doorway to Heaven. Access from Heaven to Earth from Heaven's side was achieved by simply opening a door marked "42".
Prior to Metatron's reign coming to end, he was known to move the location of the entrance around. The portal was eventually located long-term in a playground somewhere in Arkansas[2], and was always guarded by one or two angels who drew the sigil in a sandbox. As Metatron's spell was not reversed, even after his capture, this was the only method through which angels could access Heaven from Earth.
Humans are unable to pass through this gate and survive. Castiel planned to have Kelly Kline walk through the gate, killing her and her Nephilim child, whose souls would have ascended to Heaven, while every cell of their beings would return to the universe.
Metatron's Cube in Lore
Metatron's Cube is a geometric figure composed of 13 equal circles with lines from the center of each circle extending out to the centers of the other 12 circles. It is an example of sacred geometry.
