The Unification Church
Around the time of the Fourth Dragonslayer War (the fifth having taken place about 70 years ago), the Unification Church, with the blessing of the Dragon Emperor and the Council of Wyrms, began a crusade of conversion across the worlds of the Empire. Humanoid races were encouraged, sometimes coerced and manipulated, in to believing that the individual deities they worshiped were a single face of a greater entity. According to the Unification Church, there are 12 universal archetypes that all deities are a facet of. For example, the deities of far off worlds that personify death, such as Nerull, Myrkul, and Morgion or all different names for the same being - The Reaper.
Opponents of the church (mostly heretical cults forced in to hiding) argue that the Unification Church is a puppet of the Dragon Empire and is slowly killing off native gods to consolidate power.
The 12 archetypes (and their alignments and cleric domains) are...
The Destroyer (CE) - Death
The Father (LG) - Light
The Judge (LG) - Life
The Lover (NG) - Nature
The Magus (N) - Knowledge
The Merchant (LN) - Knowledge
The Mother (CG) - Life
The Reaper (LE) - Death
The Stormlord (CN) - Tempest
The Smith (N) - War
The Trickster (CN) - Trickery
The Warrior (LN) - War
Dragonmarks (from the Eberron UA article)
Any humanoid character can become eligible for the Dragonmark feat by swearing loyalty to one of the great dragon clans. (For this setting, the marks are not associated with the Eberron houses but with dragon clans, the names of which I'll provide in a later post). You must first be accepted in to the house and then, upon taking the feat, have the mark magically and physically transcribed on to you skin by a dragon. The marks serve as an indication of elite status and privilege and those bearing them are afforded great status (for a humanoid) within the Empire.