Bengü taş means "eternal stone": a monument with writing cut into it. The Orkhon inscriptions — raised for Kül Tigin, Bilge Khagan and Tonyukuk — are called this, and they are the oldest long texts known in Turkic.
Their language is startlingly direct. Bilge Khagan scolds his people, counts their mistakes, says that when you are full you do not think of hunger. It is not a victory monument but an accounting.
In Balbal, The Eternal Stones is the last floor of the Underworld and the sign that the surface is near. The reward suits the text: all twenty-four clan tamgas unlock.