The Quickbar
Good work! You can also equip and use items via the quickbar. Try opening up your inventory and dragging some items to the quickbar. Then, try opening your character sheet (the "Cha" button) and clicking on the Abilities tab. Any active ability can be dragged to the quickbar. Also note that all items and abilities have a right click menu. You can even drag items for both of your hands (such as a sword and shield) to one slot.
Once you are done experimenting, go ahead and dispatch the remaining opponents. You will note that your "Mighty Blow" ability has a cooldown, preventing you from using it again immediately.