So far, three Obelisks, linked to these demons, have been discovered scattered across the New World - one in the caves close to the village of the Watchers' Tribe (controlled by Marbas), one within the Millennium Forest (controlled by Stolas), and one in the caves close to Roanokae Camp (controlled by Agaraal).
"A column of black appears to those capable of seeing in the darkness. While it's impossible to distinguish colours with Darkvision, it just feels black."
- Bjanu
"The most eye-catching feature is the enormous black obelisk that lies within. To anyone who looks at it, it not only appears black, but feels black, as if it sucks in the light around it."
"THE CONDUIT - The 'Living Stone' & the Sigil Stones
The Goetia employ 'living stone' quarried from the deep seas, near the thermal vents, and shape large amounts of it into their 'conduits' - black, monolithic Obelisks. No runes or writing of any kind were detected upon them. This dark, resilient material is hard to access and even harder to damage, but easy to shape and form, presumably with the right knowledge and tools. Upon interrogation, Goetia Lord Agaraal revealed that he wasn't in charge of shaping the Monolith or selecting the material, so those decisions seem to be coming from the Teacher themself, or someone closer to them than even the Goetia.
It's been discovered that it is close in composition and behaviour to Sigil Stones; such (unfortunately defunct) stones have been gathered during the shadowy siege of First Landing, and used in the creation of Staves of the Eclipse. Active Sigil Stones can sustain a rift between the Material Plane and another Plane when powered properly - in the case of the invasion, it was the Shadow Plane, and in the case of the Goetia, it is most likely the Abyss.
Upon learning this, it became obvious that these Obelisks are closely related to shadow magic, and that light magic could unbalance them, if not outright disrupt them fully. Sufficient enough magic to do so is yet to be discovered, but a powerfully light artifact could do the trick.
THE CONDUIT - The Binding of an Elemental Spirit
Once the Obelisk is shaped, the Goetia subjugate an elemental spirit, transforming it from its material form into the ethereal one using an unknown ritual. Any kind would do, but according to Marbas, the Goetia use Salamanders (in the case of Demon Lord Agaraal, it might have been a Djinn instead, named Sahira). The creature must be physically present during the creation of the conduit, lest the link is not established properly.
THE CONDUIT - The Ritual
Lastly, a powerful spellcaster is needed for the ritual. The incantation lies with the Teacher - Goetia Demon Marbas claimed not to be privy to this knowledge. He did reveal that there might be other sources - independent persons, records etc, but those might be lost to time. In the case of Agaraal's Obelisk, his Genie watchdog Sahira could be a candidate for this role, instead of being the spirit that is bound to living stone, though she appears more like an arrogant, hedonistic warrior than a skillful mage."