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A Sphere of Annihilation is a magical hole in the multiverse that obliterates any matter that enters it. As part of their ritual in the Ziggurat beneath Whitestone, the Briarwoods created a marble-sized spinning sphere with properties of negating or absorbing matter and magic. Later, Vex'ahlia asked Shaun Gilmore what it could be, which reminded him of a Sphere of Annihilation. Such spheres were typically larger than the marble-sized sphere described by Vex'ahlia, but Vex suspected that this one would have grown in size if the ritual had continued.
Vex fired an Oracle Shot, using Fenthras, through the Siphon, and was able to see on the other side. The other side of the Siphon seemed to be an area of enhanced magical capabilities, due to the twin of the Siphon emitting magic as opposed to absorbing it.
Matthew Mercer described the first glimpse of the Shadowfell:
"As the arrow landed and clattered to the stonework, maybe twenty feet below the orb looking around she saw a landscape of blackened broken stone and ground. Distant mountain-scapes of jagged crags like teeth across the horizon. The sky itself was black and shadowed like it was perpetually amidst a distant forest fire. And where you can see the grey in the sky you can see just oiling thunder clouds, with flashes of lightning. And roughly a mile or so from, what you could tell, where this was, you could see what appeared to be a large obsidian tower of twisted black stone - that went up for what appeared to be a dozen or more stories." 
When Vex told Allura, the following morning of the breakthrough, what she saw on the other side of the Siphon; Allura claimed that it reminded her of a location in the Shadowfell.