Sell all the rooms after Mendacius completes his comments after you build them. One gnarling must spawn during the tutorial. To take care of any of the gnarlings that appear, Left Click to pick it up then recycle it by Right Clicking over the unit portal. After the ghosts spawn, imps will fight them off. They will automatically respawn, so you cannot simply recycle them. To prevent your imps from saving you, go to your unit list, pick up all the imps, then hold them and wait for Mendacius announce his disappointment.