Black Diamond Mine

A cave full of black diamonds. A central chamber from which branch out five narrow tunnels, carved by mining tools. Undead humanoids chipping away at the diamond tunnels, digging ever deeper and further away. Slaves to their tasks, urged forward by gemstones from another world, embedded into their skulls but willingly acquired. Mindless, full of purpose, never ending toil. Will the party free them? Will they leave with cursed treasures or become the next generation of mining drones? 

This is an encounter that is both a puzzle and a trap. Curiosity alone will not doom the party, but violence and greed will. The undead miners won't fight back against any attacks, but the mysterious crystals will aggressively attempt to possess any potential new recruits to replace them.

Summary

Environment: underground, mine, cave

Monsters: undead, skeletons

Quirks: non-combat (potentially), cursed magic items

Central Cavern

Natural cave (everything is nearly-indestructible black diamonds)

Five Holes (sounds of pickaxe, cold air, pitch black tunnel)

Table (very low and cold, petrified wood, hidden writing from forgotten civilization)

Tunnels

Each tunnel descends downwards at varying angles. Each is steeper than the last, and the walls are razor sharp. 

Descending a tunnel requires a Strength (Athletics) check at DC 13. Failing the check means that progress is very slow, awkward and claustrophobic; pass a Constitution saving throw at DC 13 or else gain 1 level of Exhaustion. Failing the check by 5 or more means that the climber slips and falls, taking 22 (4d10) damage from the fall and razor sharp walls.

At the bottom of each tunnel is a Skeleton, whose only purpose is to keep digging away.