Well, let’s start by looking at the actual kanon lore.
There are thirteen solars, each assigned to monitor and administer justice within one of Eberron’s planes (no solar holds dominion over the Material Plane). However, there are a host of restrictions on how and when they may act. Typically, a solar must be invoked by a legitimate authority within the plane in question—so while Hazariel, the Solar of Syrania, is usually called in to cast down radiant idols, Azazar, the Solar of Xoriat, has never yet been called on by that plane. Until called, they watch; while in the Panopticon, solars can observe anything that occurs in the plane of their dominion.
So one of the first important points here is that Azazar has never actually been to Xoriat, and likely never will; who would summon them? Having said that, Azazar has been monitoring Xoriat since the beginning of creation and in my mind is suited to that task; the Solars were created for this purpose and Azazar was made to be able to administer justice in Xoriat if it becomes necessary to do so.
So… Azazar is a Solar and uses the base attributes of a Solar. But they are also made to be able to observe and act within the Plane of Revelations. Solars already have truesight; I’d probably add some sort of mind blank affect to Azazar’s block, and further give them the Alien Mind trait of a daelkyr, on the basis that in order to be able to observe Xoriat, Azazar’s mind must process knowledge in ways that material mortals can’t; it’s not malevolent, but making contact with their thoughts will disrupt your normal thought process.
Moving further, I’d probably roleplay Azazar much like Doctor Manhattan: not malevolent, but as someone who perceives reality in a way you can’t understand. To be able to “observe” Xoriat and follow the progress of events there, Azazar would have to be unbound by the perception of linear time and might be actively aware of multiple possible timelines simultaneously. So I’d emphasize that Azazar CAN be a source of information about Xoriat, but that the information you receive will often be cryptic or inaccurate, because it will only make sense when you’re in the right time, place, or even the right timeline. Azazar isn’t in any way corrupted by Xoriat; Azazar understands Xoriat, but they can’t share that understanding with a creature with a limited, linear perception of reality.
Source: https://keith-baker.com/ifaq-nilbogs/