They say knowledge is power. Grady's prism will show you the most immediate realities available to all the Gray Family members from where you're all standing in the house. If you want to know the ins and outs of the Gray Family, understand better what each member is going through, or simply get a look at what you might be able to exploit, this is the safest spell you can cast. Find hidden histories, latent plot-lines, and lost tidbits of mythology. This is a totally confidential spell. No one will know you've cast it. 

Buyer beware: It's hard to forget what the prism might have to show you. Ignorance is bliss, and knowing what fates are laid out for us might be ultimately... overwhelming.