By definition, a door, the vestibule, used of any opening like a door, an entrance, way or passage, the "name of him who brings salvation to those who follow his guidance."
In the series, this emblem was discovered by Abbie when she was trapped in the Catacombs, it is the same emblem that humans used to lock the Hidden One away for millennia, and the symbol the Abbie obsessed over when she returned to the real world from the Catacombs.
In Sins of the Father, Abbie goes out to the garage and gets out her journal, and then removes a cloth to reveal the symbol painted on the wall. She opens the journal where she's written the symbol dozens of time, crosses her arms, and promises that she now belongs to it. In Dark Mirror, Abbie's obsession with the symbol almost cost Crane his life.
In the episode, Into the Wild, the emblem traps both The Hidden One and Crane within the Archive with an impenetrable barrier of energy after he arrives and aims his deadly blue energy and fires at Crane. The pendant leaps out of Crane’s pocket, catches the light, and hangs suspended in midair. The Emblem of Thura feeds on the power of the being it imprisons; if the Hidden One keeps raging against the barrier, he’ll destroy everything around him.
When Crane locks the tablets and the emblem together, it allows him an unobstructed look at what’s been going on outside the Archives. When the Witnesses devise a plan to free Crane, the tablet and emblem were destroyed when the barrier came down.