Start out by playing the Dark Maze to collect Shiny Bits as rewards. Then head over to the Maze Shop, where Alf Underhill has a random selection of items on which you can spend your Bits.
Some of the items for sale include Zibuls which can unlock, upgrade, or convert your heroes' Awaken Skills. You may also see Hero Pieces and Rainbow Sprites for crafting and evolving heroes respectively. These items come in different rarities, so be sure to double-check an item's star rating before buying!
Don't see the item you want in the Maze Shop? Refresh the selection to view new options. Who knows? That rare Zibul you've been wanting might come up! You can refresh for free every few days or instantly using gems.