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> What galaxies it connects to? Should I consider this as an "above all galaxies thing" so it can connect to any galaxies or it has fixed dimensional connectors to specific galaxies.
The short answer is that it was created to be as diverse as possible, including as conflicting beings as possible, to increase the scope of races it can be useful for. Bring it to the point when they're almost about to rip each other's throats out, but they realize that "if we cooperate for a session right now, our combined energy will make us all stronger, and it's more beneficial to just follow the ritual instead."
The sigil shows the "home" galaxy as the yellow line, and the other 4 are there to create the 5 points. When scaling up a circuit you can do a circle which is just eternal, a 1. Then you can create a triangle, 3 points, and next is a pentagram, 5 points. The pentagram is better suited for large circuits repeated fractally, so that was used.
> What other entities partake in it's construction?
A variety, enough to represent the intent of "diversity creates strength" (as long as we contain our hostilities). There were vampires, demon lords, drow, yokais and so on, one for every race available at the time of formation. The collective egregore sort of demanded this exist, so we had to make do with who was available when the energy started forming.