Gods, Celestials, and Fiends

The people of Eberron believe their gods are omnipresent—not bound to a single coherent form, but present in all places. If you revere the Silver Flame, its power is always with you. The Sovereign Onatar guides the hand of every smith who knows how to listen for his voice, and Dol Arrah and Dol Dorn are active on every battlefield, guiding every soldier. This outlook means that religion is driven by faith, as opposed to the concrete actions of deities. The faithful believe that their triumphs reflect the assistance of a divine influence. They don’t expect a god to physically show up and solve their problems.
Fiends and celestials certainly do have physical form, however. Fortunately for the folk of the Material Plane, these extraplanar creatures are deeply invested in their own affairs and have little interest in Eberron. Demons and angels battle one another in Shavarath, but they’ve been doing this since before humanity existed, and they dare not leave their posts to fool around elsewhere. Exceptions do exist, such as the daelkyr and the Dreaming Dark, but by and large these natives of other planes are exclusively concerned with where they live. As such, player characters are more likely to encounter celestials and fiends that are native to Eberron, spawned by Khyber or Siberys, rather than extraplanar entities.
Couatls Couatls are a celestial race born from Siberys at the dawn of time. Along with the dragons, they battled the fiendish overlords of Khorvaire and Sarlona. Ultimately, the couatls sacrificed most of their number and combined their souls to form a prison to hold the overlords. Scholars have theorized that their combined souls eventually became the force that is worshiped by the Church of the Silver Flame. The church is ambivalent on the topic, preaching that regardless of how the Flame was first kindled, there is a place within the Flame for all noble souls.
The few couatls that remain on Eberron are devoted servants of the light. They are most often found guarding the prison of the ancient fiendish overlords, and individuals sometimes act directly to aid adventurers who fight the forces of darkness.
Fiends The mightiest of the fiends born from Khyber are creatures of immense power known as the overlords. Dozens of overlords were imprisoned by the Silver Flame at the dawn of time; two of them, Rak Tulkhesh and Sul Khatesh, are detailed in chapter 6. Another—Bel Shalor, the Shadow in the Flame—is bound within the Silver Flame itself in the great cathedral in Flamekeep in Thrane. You can adapt other evil gods or archfiends for an Eberron campaign (assuming you don’t want Eberron to be connected to the wider multiverse) by recasting them as overlords. For example, Tiamat could be an overlord embodying the pride and potential for evil within dragons, Lolth could be an overlord who preys on the elves, and Asmodeus the insidious maker of profane bargains.
Any fiend—whether devil, demon, yugoloth, or something else—could be spawned by Khyber if it suits your story. Such fiends might be bound to an overlord, or they might be independent incarnations of evil unleashed on the world. Two other races of fiends, though, play important roles in its history.
Night Hags. Night hags have been around since the Age of Demons, when they often served as ambassadors and carried messages between the fiends and the dragons. Today, they remain impartial mediators, and adventurers who are about to deal with outsiders or other planes of existence might want to seek the advice of a night hag—although one can be quite difficult to find.
Rakshasas. As the primary agents of the fiendish overlords, rakshasas once dominated Khorvaire and Sarlona. When the overlords were defeated in the first age of the world by the combined might of the dragons and the couatls, the rakshasas largely disappeared into Khyber as well. The Lords of Dust are made up largely of rakshasas, scheming in the shadows—many trying to free their imprisoned overlords, others looking for a way to claim their masters’ power for their own.