Ryan Silva asked for “long term and short term effects of subterranean adventuring if you would.” I would and could and can and did.
Fatigue and vitamin deficiency.
Disturbing and colourful hallucinations become common whenever the lights are out. They are clearly not real though they do prevent you from sleeping.
You begin to know where you are in the cave when you wake up. You have recall of nearby formations without turning on the light.
In the first weeks, paranoia and rages are common.
During the first six months the rapture stalks you, after this, if it fails to take you, it disappears and comes only occasionally.
You slowly abandon useless items, ruthlessly purging your belongings for weight. You shave your blades to razors and slice from the soles of your shoes.
As the memory of the sun decays your claustrophobia lessens. The day sunlight disappears from your dreams, you no longer fear small spaces, even the narrowest squeeze can be pushed through without a test.
You recognise worked passages from the tool-scars left. Later, you can tell the size and natre, and mood, of the worker by the trace of their swing.
You come to recognise the identity of spilt blood by its smell and taste, if the person is known to you.
Maximum pupil dilation at all times.
Gradual, then total, pigment loss from skin.
In the first week, a total collapse of your immune system. Immunity reaches zero. Then invisibly replaced with a (relatively) harmless semi-intelligent fungal symbiote.
Skin becomes extra-sensitive, clothes uncomfortable. After weeks you can learn to use your skin to sense the air flow in the surrounding area, giving you a good idea of cavern size and possible exits. As well as general size of linking passages.
Hearing becomes extra-sensitive. You subconsciously begin making tongue clicks to judge distance in the dark. Though unaware of this.
The rhythm of your conversations slows imperceptibly. You breathe out single words. Shouting becomes intolerable, even in times of danger. Battles pass in silence on both sides.
Osteocytes begin to form upon you scalp and joints. Harmless, painless frills and horns of bone.
Your finger bones lengthen and crook permanently. This provides a tireless hold while climbing and increases your ascent speed.
Your time sense erodes without any realisation, your wake-sleep cycle extends from 24 hours to 48 then to 72. You do not notice this
Respiration and digestion slow. In silent moments or when still you sometimes go 30 seconds between breaths.
Your breathing becomes non-audible when you are sleeping or waiting.
You no-longer trust paper as it decays too fast. You start to tattoo your knowledge and maps onto your skin. Your self-developed code is shorter than writing and means your flayed skin would be useless as a map for others. A clever trick, you think. You start with your forearms and thighs then spiral out.
Effects on Magic Users
Casting of Light spell changes gradually, darkens to infra-red, then becomes nearly invisible as memory of sunlight decays.
Surface creatures gradually disappear from the results of summoning spells.
Any fire generated stats to burn deep-blue.
ESP no longer brings you visions of others mental states. Only flows on smell or movement, taste and sound, you do not notice this.
Spells used to trace and spy on others likewise lose their visual component, you know where someone is, you smell, taste and hear them. You feel them in the air. You do not see them. This is not disabling in any way.
Spider Climb lasts longer. It begins to eat up spell slots without you realising. When all 1st level spell slots have been consumed, Spider Climb becomes semi-permanent, subconsciously and cyclically cast and re cast whenever you are awake. Following this, the concepts of ‘up’ and ‘down’ will begin to lose all meaning. You may not understand the words.
The Imprison spell begins to feel shaky and unreliable. You begin to think you hear the voices of people you had previously imprisoned as you near their location.
The spell Flight begins to decay. It lessens, disappears, then returns as ‘Dive’. This lets you dive and walk through rock as if it were water or gas.
Effects on Clerics
As you approach Hell, the reach of heavean comes less frequently, silently but with invisible force. You can burn spell slots for one of a kind high level miracles.
Gods voice intensifies as you reach the Core.
God speaks continually now. But unaware of you. As if addressing a crowd in the next room.
You can no longer create food or drink. Yet the same spell allows you to fast for days without any effect.
The earth regards you. The stone is aware. You know that if you addressed it, it would hear. Yet this is wrong and forbidden.
The stone is one with the stars. You can read the curls of sediment and speleothem as if they were hidden constellations.
For a long time lightning does not heed your call, then, as you grow deeper, suddenly it does. This time coming from below.
(A few of these are inspired by Jeff Longs book The Descent. If you gave that book an index and some sub-par illustrations it would be half way to being a module on its own.)