A few metres from the trilithon stands a large upright slab of coral limestone, shorter and rougher than the Ha'amonga's pillars but striking in its own right. Known as the Niu Stone, it is said to be the ceremonial backrest of the Tu'i Tonga — the sacred king who ruled Tonga around 1200 AD and for whose compound the trilithon formed the entrance. According to tradition, the king would sit with his back against the stone, facing outward, during formal court gatherings.
The purpose of the Niu Stone is debated, as it is with most of the site's features. It may have been a boundary marker, a ritual seat, or simply a natural outcrop worked by human hands. What is clear is that it formed part of the same royal landscape as the trilithon — a landscape designed to project permanence and power. Standing next to it today, with the trilithon visible a short walk away and the sea not far beyond, it is easy to imagine the compound as it once was: a place built to last longer than the kings who ordered it.