"You are right to fear the Dark Reflection. Every terrible impulse, every twisted ideal, every dark desire hidden from your peers can be seen in brief glimpses on the other side of the mirror. But as much as you fear the Thirteenth, you should still pray to him. Pray that you can master the dark side of your soul, pray that you can be better than all that you fear, and pray that He can save you from what you fear you might become."
The Pantheon represent high ideals for the faithful to aspire to, whether it be the charity of Gild, the chivalry of Gloria, or the wisdom of Vellichor. The Thirteenth, Tehom, He Who Waits Behind the Mirror, the Dark Reflection is the personification of the opposite of the ideals, and according to the old orthodox of the Faith in the Oathlands, the ruler of the Abyss and demonkind. His very presence in the Pantheon is contentious, as many of the more stringent and conservative members of the Faith of the Pantheon (particularly Valardinian faithful) believe that his worship should be banned, knowledge of him should be eradicated, and any association with him invites in evil to corrupt and perverse the virtuous. Most of the commons of Arvum likewise are extremely superstitious where the Thirteenth is concerned and very few commoners own a mirror, with reflective surfaces being considered poor luck.
This is not to say that worship of the Thirteenth is prohibited in any way. In the south, the Lyceum venerates Tehom, believing that the Dark Reflection allows individuals to channel their passions and control their impulses to become stronger individuals. Bottling up the negative reflections of virtues and refusing to acknowledge that sin exists only invites higher falls from grace, they believe, and a libtertine permissiveness exists throughout many of the southern city-states. The discipleship of the Thirteenth, called Mirrormasks for the reflective masks they wear, are uncommon outside of the Lyceum, but a small number are found in Arx to the discomfort of many. Often mistaken for voluntary mirrormasks are the Silent Reflections, members of the Faithful guilty of great crimes (such as heresy) who instead of execution are sentenced to lifelong servitude with their tongues removed and a writing hand cut off. Few priests of the Pantheon ever hold services for the Thirteenth, so traditionally only mirrormasks hold the sparsely attended (outside the Lyceum) services.
The Thirteenth has only a single feast day, celebrated once every few years during an astrological event known as the Eclipse of Mirrors, a night when a bloodmoon is visible and it is said reflective pools tend to be far more clear. In the Lyceum, House Velenosa started the tradition of hosting the Mirror's Masquerade, a celebration in honor of the Thirteenth where an actor is hired to portray the dark god as the guest of honor for a night of revelry. While House Valardin very pointedly does not celebrate the Eclipse of Mirrors, most other houses do indulge in the carnival atmosphere of the night, though not to the extent of the nobility of the Lyceum who are known for spending outrageous sums in cutthroat competition for who might be considered the most fashionable and memorable on the night of the Eclipse.