The Infinite Sea. It closely resembles outer space in Aurae, but is, at times, awash with color. There are ribbons of light that move throughout the void, and occasional bobbing globes of light, rather than stars. The truly mystifying aspect of this realm is that the spectrum of light and color does not correlate with any known spectrum in any other Realm. The array of colors is entirely different, and tends to be overwhelmingly confusing to the uninitiated mortal.
Locomotion, perception, survival, and life itself is purely derived from psychic energy in this Realm. What can be willed into existence is what will be manifested. Appearance here can vary radically from visit to visit, and encounter to encounter. Sometimes a Native will have created a mini-realm, only to eradicate it decades (or mere moments) later. In fact, it is normal to never encounter a Native or traveller, or any structure or construct of any kind whatsoever. This is due to the infinite nature of the realm coupled with the very low population. However, since the realm is so delicately attuned to psychic energy, it is possible to "sense" other beings from any distance, if psychically enlightened.
It is debated that this realm is somehow closer to Primo Regni than the others, and the denizens have a heightened capacity for creation and destruction. Unfortunately, most non-Natives lack this understanding, or the imagination to harness anything from it, and perish almost immediately upon entry.
Though various Excubiae have different perspectives on the birth order, they may all be correct. Time is a fluid concept, especially with regards to what comes from Primo Regni. For Advena's part, It had always been here. It's rumored that since the moment after Z'zhuul's self-inception, there has been Advena, and that was when time began. Where Z'zhuul is the embodiment of chaos - pure construction, pure destruction - Advena is Its counterpoint. Measured discipline and minimalism are Its hallmarks.
Next came the Two, Jehovah and Lilith. Advena perceived Their birth to be simultaneous, and thinks of Them as twins. It hovered over Them as They created their Realms, faintly satisfied with having "siblings", but found Them impetuous and emotional. Resolving to leave Them to make their own way, free of Its intercession, Advena returned to tending to Its Realm. It was progressively less attuned to each successive Excubiae: It found Uthrasa much the same as the Two; Puer Hostiæ was an innocent, but ultimately unengaging babe and the birth of Carnifice was forgotten almost as soon as it registered.
Resembling a titanic, translucent and ever-changing jellyfish, Advena possesses many strange, alien appendages and sensory organs. It's never addressed itself with a name, but Its siblings took to calling it "Advena" early on. For unknown reasons, long ago, Advena split off forty different versions of Itself, all smaller, but identical in appearance. All with the same limitless shape-changing ability to take any form they choose. It is unknown if any of these versions have ever had occasion to re-mesh with one another, or if Advena plans to absorb them back into Itself. Uthrasa has hypothesized this event as being an attempt to experience more input at any given time, but even She admits She knows not why there were only forty changelings birthed, or why a near-omniscient being would need multiple perspectives.
Though no real communication with Advena has ever taken place, the Excubiae are able to read each other to a certain degree, even one as alien as this. The general disposition that has been gathered on the few occasions It was encountered was a detached benevolence. The subject of infighting, machinations and intrigue between the various Excubiae elicits a more contemptuous reaction, however. Mostly, Advena doesn't interfere and exists within the boundaries of Its Realm. One notable exception to this was the death of Puer Hostiæ: Advena appeared at the gathering of the Excubiae, floating at the periphery, saying nothing. For the first time, the waves of emotion coming from It were an overwhelming sadness and disappointment; Advena does not often encounter death. Then, after a few moments, Its respects paid, Advena simply faded from view, and has never been seen since.