Most leupaks (those created from venom transformations are the exception) possess a crystalline third-eye structure upon their foreheads.
This is the Facet; a small piece of the Mirajin crystal which true leupaks are born from, similar in structure to the crystals formed on Boralion dragons.
Facets contain a leupak's vital essence-- in other words, the concept, thought or idea which they sprung to life from. Touching the facet of a living leupak with bare skin will flash such imagery through one's mind.
Each individual leupak's facet may be different; while basic shapes are common, many reflect the theme its leupak is based upon.
Facets buzz softly with energy, and make powerful foci in magical and alchemical endeavors, especially those involving dreams, chaotic magic, or a subject linked to the facet of the leupak in question.
For this reason, facet-crystal is a rare, highly-prized component in all manner of magical work. A facet cannot be removed from a leupak without killing it, however; the external portion of a facet is only the tip.
The structure extends through a leupak's skull, rooting into its brain.
For shardborne leupaks, the facet is the most vital of all organs; a leupak's heart may be pierced or its brain removed, but the facet contains the thought that brought that leupak into being.
A simple fracture can render a leupak permanently insane; removing or breaking the facet will slay it utterly.
Like the leupaks themselves, the facet comes in many shapes and sizes, typically found atop the head and most commonly in the form of a small crystal. This is far from the only configuration, however-- just the most common.
In addition, a leupak's facet often grants its wielder a special ability or power befitting the concept it was created upon.
This power remains intact should the facet be removed-- making some leupaks desirable targets for poaching.