Access the Characters menu, where you'll see your items, supplies, skills, and goals. Near the upper right corner of the screen, you'll see a Gear icon. Tap that and choose the "Chair Store" option. Inside that store, scroll to reach "Imagine Dragons : Monster" and then select the "Gift" option. You'll load a site in Safari. Wait for it to load, then close it once it finishes. When you return to Infinity Blade III, you should find the Imagine Dragons Axe added to your Items inventory.