Obtained: After Mamori reaches Stage Rank 10 and you reach the Chapter 5 Intermission, you will receive a Topic from Mamori. Mamori wants you to meet her in Daitama so head on over!
Tiki needs an ore from the Illusory Daitou TV Station to make the pairing knife for Mamori. After the cutscene, head to the Idolasphere and teleport to Film Set B: Indoors (1F). Take a right and head down the hall. Exit through the door at the end of the hall and take an immediate right. Follow the path forward towards the stairs leading up. Instead of taking the stairs up, head through the door to the right of the stairs. The Illusory Obsidian will be located on the ground diagonally left from you next to stacks of boxes.
Once you retrieve the item, travel back to Bloom Palace and give the Illusory Obsidian to Tiki to trigger a cutscene. Afterwards, return to Bloom Palace and speak with Tiki to obtain Mamori’s new Radiant Skill “Door of Dreams”, which will complete the side story.