In the slate are a number of runes with powers. (vooks.net)
Once you fall down from the bridge into the Marketplace Ruins, defeat the Draugr, and Atreus will likely go to inspect a large column nearby that's filled with runes of lore. (au.ign.com)
Further to that, it's got a leather touch pad cover with the signature God of War logo and also lights up with ancient runes on the back. (press-start.com.au)