Tul Oreshka, the Truth in the Darkness, is one of the Overlords who ruled over all of Eberron during the Age of Demons. Tul Oreshka was bound by the dragons and couatl after they jointly discovered the Draconic Prophecy, and the couatl sacrificed themselves to form the Silver Flame.
All of the Overlords embody mortal fears. Tul Oreshka embodies the fear of terrible secrets, things we want to know but know that we shouldn't, the secrets we know will hurt us in the end. Tul Oreshka knows what your closest friends really think about you, the terrible things that they've done but kept from you, the exact moment and method of your death. We aren't built to know everything, and Tul Oreshka teaches us this lesson harshly.
Her influence drives others to madness, as she whispers the things they fear to know in their minds. She is unpredictable, and for this reason, Tul Oreshka is as much of a threat to her allies as she is to her enemies.