The Heart of the Giants is a Key Item in Xenoblade Chronicles. It is one of the three treasures of the Giants, the other two being Truth of the Giants and Daring of the Giants. These three items are required to open the door at Three Sage Summit, and are required for The Giants' Treasures and The Final Giants' Ruins.
The Heart of the Giants can be obtained by completing the following chain of quests which become available after completing Kind Lupa's Grampypon and Secret Elixir Ingredients: