Access the Characters menu where you can view your items, supplies, skills, and goals. In the upper right corner of the screen you'll see a Gear icon. Tap that and select 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.