Mythic / Fantasy MythicFantasySubterraneanSunUnderworld

The Subterranean Sun Keepers

Fueling the Miniature Sun in the Under-Realm

By Alex Merced · Published 2026-08-03 · Online Supplements

Five miles beneath the surface lies Sol-Prima: a vast subterranean cavern city illuminated by Sol-Core, a glowing, miniature artificial sun suspended in the cavern center. A holy order known as the Sun Keepers maintains Sol-Core, feeding it rare radiant-crystal fuel to simulate day and night cycles for the underground civilization. But the radiant-crystal mines are running dry, and Sol-Core has begun to flicker, casting long, unnatural shadows where shadow-beasts materialize.

Core Mechanic: Solar Radiance: Track Sol-Core's brightness and energy level as Solar Radiance from 6 down to 0. Failing to stoke the core or losing crystal fuel loads reduces Radiance. At Radiance 1, permanent twilight sets in. At Radiance 0, total darkness triggers shadow-entity invasions.

“If Sol-Core goes dark for more than ten minutes, the Shadow-Crawlers come up from the deep shafts. Keep the furnace fed.”

  • High Keeper Solon

1. Setting Overview and Tone

Sol-Prima is a subterranean metropolis of stone bridges, terraced gardens, and glowing crystal towers built around a central chasm.

Suspended over the chasm is Sol-Core: a 100-foot sphere of golden plasma contained within brass magnetic rings.

Tone: Heroic, luminous, atmospheric fantasy. High contrast between blinding holy light and terrifying subterranean darkness.

Sensory Pillars:

  1. Sight: The brilliant golden glow of Sol-Core illuminating multi-tiered stone streets. Long, dancing shadows in outer cavern districts.
  2. Sound: The deep, warm hum of the plasma core. The ringing of shift bells signaling the artificial “dusk.” The crackle of radiant crystal fuel in the furnace.
  3. Touch and Smell: Intense heat near the furnace towers. The clean, dry smell of ionized air and burning crystal.

2. The City Districts

The Sun Tower: The central spire housing the crystal loading machinery and magnetic containment controls.

The Terrace Gardens: Bioluminescent crop terraces lit by Sol-Core’s daily light cycle.

The Shadow Outskirts: Outer cavern neighborhoods where light is dim and shadow-barriers are maintained by local guards.

3. Factions

The Order of Sun Keepers: Priests and engineers who manage Sol-Core’s fuel intake and magnetic containment.

The Crystal Miners Guild: Deep-shaft miners who extract radiant-crystals from dangerous lower caverns.

The Cult of Eternal Night: Outlaw zealots who believe Sol-Core is an unnatural prison for true subterranean shadow entities.

4. Unique Mechanic: Solar Radiance

Track Solar Radiance from 6 down to 0.

Radiance States:

  • 5-6 (Full Noon): Bright illumination across all districts. Plants grow; shadow entities cannot manifest.
  • 3-4 (Dusk): Dim light. Outskirts fall into shadow; minor shadow-crawlers appear near city walls.
  • 1-2 (Eclipse): Deep twilight. City-wide panic; major shadow entities breach outer districts.
  • 0 (Total Darkness): Sol-Core extinguishes. Full shadow invasion; life support collapses.

5. Character Archetypes

The Crystal Stoker

You handle heavy tongs and heat-shield armor to feed radiant-crystals into the core furnace.

  • Drive: “Keep Sol-Core burning no matter the cost.”
  • Approach Distribution: Force 3 | Finesse 1 | Mind 1 | Presence 2

The Magnetic Engineer

You adjust the giant brass containment rings that suspend and stabilize Sol-Core.

  • Drive: “Prevent plasma containment failure.”
  • Approach Distribution: Force 1 | Finesse 2 | Mind 3 | Presence 1

The Shadow Guard

You patrol the dark outskirts armed with radiant-crystal lances to defeat shadow beasts.

  • Drive: “Protect the city perimeter during dusk cycles.”
  • Approach Distribution: Force 2 | Finesse 2 | Mind 1 | Presence 1

6. Starter Scenario: The Crystal Shortage

The primary radiant-crystal mine suffers a collapse, cutting off fuel supplies to Sol-Core. Radiance drops to 2 (Eclipse), and shadow entities begin breaching the lower terrace gardens.

The player characters must lead a desperate expedition into the collapsed deep mines to retrieve emergency crystal reserves, fight off shadow beasts in the dark, and return to feed the furnace before Sol-Core goes completely black.