Few escaped the fire of the dragon Smaug when he brought death and ruin upon the dwarf realm of Erebor. Centuries later, the precious key to this mountain kingdom's back door would find its way into the hands of Thorin Oakenshield, heir to the Erebor throne. Keys can unlock and lock things both physically and magically. Keys can symbolize being initiated; give access to arcane knowledge and, as you know, knowledge is power.