There are a number of special items needed for learning some Knowledge in the Oracle.

A Totem is an item needed for learning Knowledge in the Tier VI Troops category. You can get it for passing checkpoints in Competitions and for taking top places in the League of Conquerors, League of Masters (in clan Competitions), and League of Odin (in personal Competitions) rankings.

Obsidian is an item needed for learning Knowledge in the Tier VII Troops category. You can get it as a reward for passing checkpoints in Competitions and taking top places in the League of Asgard rankings. You can also find it in Coffers of the Gods, which drop when you yield resources in level 8 and 9 Gifts of the Gods resource locations.

A Manuscript is an item needed for learning certain Knowledge in the Oracle. You can get it by completing Personal Tasks or purchasing it in the Clan Store for Loyalty Points. You can also receive Manuscripts by attacking Uber Invaders. The chance of getting a Manuscript depends on the Hero's Skills, learned Knowledge, and equipment and boosts that increase the Hero's military stats during fights with Invaders.

You can purchase all the items mentioned above as part of Bank offers, too.

The Totems, Obsidian, and Manuscripts you have are displayed in the Resources section of the My Items tab. They cannot be applied from the Item Store or the My Items tab.