Tehom Dogma 1
"If thirteen wise men and women of honor and faith are in a room, and twelve agree upon the rightness of a cause, the persuasiveness of an argument, or the safety of a path forward, then the thirteenth must argue vehemently to the contrary. No truth, however elusive or unconvincing, must escape us due to a lack of scrutiny. The signs of the Reckoning were before us, and we neither saw nor heard, convincing one another we were safe. We failed to see the warnings once. We will never fail again." --- Dominus Marach, in his address to the Knights of the Library
All the gods are worshipped in turn when a member of the Faithful feels the need to pray upon an aspect of the Pantheon. They might pray to Gloria for valor in battle, to Vellichor for wisdom, for Limerance for the strength in keeping their vows, or Jayus for inspiration. When then, do they pray to the Dark Reflection, the Thirteenth? For most Arvani, it is when they have cause to doubt. Worship of the Thirteenth is about questioning one's suppositions, and looking at any ideal from an entirely different angle to try to discover any truths that can lead someone to a more virtuous path.
The Dark Reflection is about finding exceptions to the deeply held ideals, for when all encompassing ideals might lead to tragic outcomes, or when otherwise moral actions might lead to immorality. At its heart, the inclusion of the Thirteenth in the worship of the Pantheon is why much of the population are intensely skeptical, and why the Oathlands Orthodox of the Faith disputes its findings. Fundamentally, the Dark Reflection teaches that any faithful should always look at any belief from other angles and reflect upon the closely held belief, and only by passing close scrutiny can the truly righteous path be discovered.
All the gods of the Pantheon are essentially ideals or idealized aspects of the world that benefit humanity. The Faith of the Pantheon recognizes that there will always be flaws in those ideals, and cases when those ideals have exceptions, and that's the nature of the Dark Reflection, are a reminder when those exceptions are necessary, or even sacred. It's not uncommon for a priest of the Pantheon when questioning a decision that appears to be righteous, but could lead to tragic outcomes, to pray to both Vellichor for wisdom and to the Thirteenth to help her see the problem from every angle. The Dark Reflection represents the necessity of a contrary opinion in order to have a more complete grasp of any issue.
Mirrormasks, then, are those who specialize in being contrary, and see themselves in many ways as the guardians of the minority opinion, to make certain that viewpoints are never completely extinguished or lost in the passions of the day. Most mirrormasks are advocates for the defense at trials, and priests of the Pantheon specifically try to assign a mirrormask who firmly believes in the guilt of the defendant in order to test their faith by giving the most impassioned, powerful defense they can muster.