Obsidian Spire of Aethelgard

The tower is perched atop the highest peak of the Aethelgard Mountains on the surface of Pearl, third Moon of Aquatica, overlooking the sprawling coastal city of Veridian Bay and the vast, often turbulent Azure Sea. The summit is very difficult to reach, often shrouded in mist and buffeted by fierce winds, making the Spire a distant, almost mythical sight for the populace below.   The spire is a singular, monolithic structure, seemingly carved from a colossal shard of jet-black, impossibly smooth rock. It rises to a dizzying height, tapering to a needle-sharp apex that often appears to pierce the clouds. Its surface, while smooth, isn't uniform; intricate, almost imperceptible geometric patterns are etched into its dark facade, visible only under specific lightning conditions or from unique angles. They aren't decorative, by a complex array of conduits and receptors.   Its base widens slightly into a circular plaza paved of the same dark stone, surrounded by smaller, but less imposing structures that serve as observatories, living quarters, and power conduits. These and other ancillary buildings are there to support the spire in its function. There are gears, levers, arrays of shimmering crystals, spiraling magnetic coils, and conduits that hum with a faint, almost imperceptible energy.   The Spire is not a telescope, but a colossal, multi-spectral, subspace-sensitive resonance chamber. Its primary function is to detect and amplify the unique, interwoven signatures of the Heart of Xylos.

1. Magical Radiation Signature: As the Heart of Xylos travels through subspaces, it emits waves of exotic, magical radiation. The crystals embedded within the Spire act as highly sensitive receivers, resonating with this energy. As the rogue star approaches, these crystals begin to glow with an internal, ethereal light, starting from the deepest chambers and slowly ascending the Spire.
2. Magnetic Field Anomalies: Pulsars possess immense magnetic fields. Even mostly in subspace, its magnetic footprint causes minute distortions in the local spacetime and Aquatica's magnetosphere. The spiraling metallic coils inside the Spire are designed to pick up these incredibly subtle shifts, translating them into pulses of light and low-frequency vibrations that travel up the structure. 
3. Gravitational Ripples (Subtle): While the physical effects are "barely felt," the star's immense mass still causes minute gravitational ripples that propagate even through subspace. The Spire, acting like a colossal pendulum or accelerometer of incredible sensitivity, is designed to register these minute gravitational stresses.
4. Phasing Signature: The act of the pulsar shifting in and out of subspace creates a unique "phasing" signature. The etched patterns on the exterior of the Spire are not just conduits for energy, but also interferometric arrays that can detect these subtle distortions in the very fabric of reality. 

The Public Misconception For centuries, the people of Veridian Bay have observed the faint, intermittent lights that sometimes flicker along the upper reaches of the Obsidian Spire. Ancient prophecies and myths speak of "The Azure Comet," a celestial harbinger that brings good fortune or great change, appearing every "great cycle" (roughly 200 years). The light on the Spire is simply seen as a confirmation of its approach. Scholars have dismissed the detailed ancient astronomical charts found near the Spire as fanciful interpretations of meteor showers or rare comets. No one alive truly understands that the Spire isn't merely observing a "comet," but actively tracking the return of a truly dangerous, reality-warping entity from beyond their conventional understanding.    When the light reaches its peak, bathing the summit in a ghostly glow, and the faint, low hum from within the Spire becomes almost audible to those living closest to the mountains, it signals the closest approach of the Heart of Xylos- a moment of wonder and fear for the few who are aware of the ancient legends, but one of silent, cosmic peril for all.