Hey, thanks for your response.
I already went back to the Dark Realm and beat Umbra. After that I was able to enter and finished the quest.
I was just confused because I was able to enter the chamber to get the Gengarite without beating Umbra in Dark Realm and the NPC told me I could enter at any time.
However, thanks for your help. As my problem is fixed already, you can close this thread.
Greetings~