Lost Scriptures can be obtained from Elite Lootboxes, and can drop rarely from alert bosses. There is currently two available. They cost 5000 gold to activate, and are consumed upon use.

Lost Scripture #1

This scripture allows the use of nodes on marks on a character. Once this has been used on a character, go to the nexus and click on the bottom right image on the UI located in the top left corner. This will bring up the mark shop UI.
From this UI, you can purchase marks for Onrane. You can purchase a total of four small marks for 15 Onrane each and a single big mark for 40 Onrane. This will cost a total of 100 Onrane.

Lost Scripture #2

This scripture allows you to gain an additional 10 points in all stats through the use of stat vials. These can either be obtained as drops from raid bosses or crafted using two greater stat potions in the Sor Forge. Crafting them this way will cost you one Onrane per vial.