When the old Goddess of Magic, Mystra, was first killed, and the Magic portfolio was split in all different schools, either by fate or luck, the shard of illusion appeared directly in front of Tymora. As Iriana killed every single god, Tymora stayed hidden, always able to escape Iriana's power. However, after the war against the gods ended, she kept herself hidden.
Even in the height of the First Era, when wars were no more raging, Tymora kept herself hidden, appearing as merely illusions of different creatures, every single time as a different person. She keeps this up even to this day, and many people even question if she is still Tymora.