The Gateway to the Shadow World (alternatively the "Lady's Gateway" or The One-Sided Door) is the method by which Lucrezia Mongfish dispatched/banished an army of her Geisterdamen followers from their home (wherever that is) to Europa to search for the kidnapped Holy Child. Soon after the exodus was completed, the gateway was destroyed by disbelievers among the Geisterdamen, most notably the Loremistress Milvistle, stranding the Ghost Ladies in what they call "the Shadow World",
The gateway and plans, burning
In the aftermath of the damage, the High Priestess Vrin was able to (sort of) reconstruct the Summoning Engine with Prince Aaronev's help[1], but was not able to rebuild ✣ the gateway to return to her homeland. Quite obviously, the techniques for building shk-mah generators (Hive engines) was also retained in some way.
It is very eventually revealed ✣ that the gateway was in fact one of the Queen's Mirrors, which Lucrezia had to rebuild following the assault that led to the aforementioned kidnapping. It is still unknown where exactly in Europa the Geisters emerged from the target Mirror.
Semi-Canonical Information[]
According to various sources outside the actual comic, which may now be outdated, the Geister homeland is located near the secret base and summer home of the Mongfish family. indicating there is some other way to reach the place besides using a Mirror..