Adventurers cannot usually see the Astral Plinth; the door opens only when the Serpent Generals' seal on the hall's door is broken, and the doors open, during a troubled Besieged event. If the Hall is open, you can target the Astral Plinth as an NPC, and its health indicates how close the beastman army is to capturing the Astral Candescence. The only glimpse most players get of the Plinth is at the end of a Besieged event; when the army is victorious, a cutscene is played showing the Astral Candescence still glowing atop the Plinth.