The Stag is, simply put, a gigantic Stag Beetle who acts as Jakkal Un's avatar and companion on earth. The Stag's purpose is to appear to those deemed worthy, and gift to them +10 luck. The presence of the Stag can be detected, or at least his incoming arrival, but the deep, ground-shaking hum he makes whilst flying.
Relationship to Jakkal Un
In all honesty the Stag, whilst acting as avatar and guardian of Jakkal Un and his wisdom and treasures, is often used as a form of transport for Jakkal Un to arrive on. The origin of the Stag is debated; some say it came into being when Jakkal Un first landed on the new Eden, bursting from the dirt beneath his feet; others say Jakkal Un created it on a day when he had to walk his walk but had also deemed a mortal worthy of his luck.