Dreaming of exploring a house often symbolizes your mind and personal exploration. Each floor can represent different levels of consciousness or aspects of your life. The fact that the floors got darker as you ascended might suggest that you're confronting deeper or more troubling thoughts and feelings as you delve further into your psyche.

The dark presence in the fifth-floor room could signify fears or unresolved issues that you may be avoiding. It’s significant that your cousin mentioned a family history connected to this room, which could reflect inherited fears or anxieties that you might be carrying from your family's past.

Your grandfather's experience with "seeing demons" could symbolize inner demons or struggles that have affected your family. This dream might be prompting you to explore these fears more closely, whether they are yours or a legacy from your family.

Consider what areas of your life you might be avoiding or what fears you have not yet confronted. This dream serves as an invitation to face those issues, rather than letting them linger in the dark. Reflect on what the dark presence represents for you personally, and think about how you can bring light to those fears.