Underdark Rules

Here's how rules for Underdark D&D, from the players' perspective. I'll post more from the DM's perspective soon.

You have three choices, which can be made in any order.
1) Pick an attribute that you excel in.
2) Pick a class.
3) Pick a race.
Then figure out your To-Hit Rolls, Hit Points, Armour Class, weapon damage, and spells.

Attributes

Your character has one High attribute, two if you're human. You get a bonus to some rolls depending on your High Attribute.

Strength (Str) +1 on Melee To-Hit Rolls and Damage Rolls
Intelligence (Int) +1 to Search Rolls and Wizard Spells
Wisdom (Wis) +1 to Saving Throws and Cleric Spells
Dexterity (Dex) +1 to Missile To-Hit Rolls and Armour Class
Constitution (Con) +1 to Hit Points per level
Charisma (Chr) +1 to Parley Rolls

Classes

Fighter
"Beware the man who loves battle"
Hit Points: 1d10 per level
Best To-Hit rolls
Can use Small, Medium, or Large Weapons, Chainmail and Shield
Cleave: if you kill an enemy with a melee weapon attack, you may immediately make another attack against an enemy within reach

Cleric
"Hell Awaits"
Hit Points: 1d8 per level
Average To-Hit Rolls
Can use Small Weapons, Chainmail and a Shield
Turn Undead: Roll 4+ on D6 (+1 for Wis) to make an Undead enemy cower or flee.
Divine Spells: Cast a limited number of spells. Roll 4+ on D6 if necessary (+1 for Wis). Check spell chart for spells per level.

Wizard
"Klaatu...Nerada..."
Hit Points: 1d4 per level
Poor To-Hit Rolls
You can use Small weapons but no armor.
Arcane Spells: Cast a limited number of spells. Roll 4+ on D6 if necessary (+1 for Int). Check spell chart for spells per level.

Thief
"Without the ties of kin, folk turn to banditry and murder"
Hit Points: 1d6 per level
Average To-Hit Rolls
You can use Small weapons and leather armor.
Burglary skills: Roll 4+ on D6 (+1 for Dex) to pick locks, find traps, and disarm traps.
Use Scroll: Roll 4+ on D6 (+1 for Int/Wis) to use an Arcane or Divine spell scroll.
Backstab: +2 to hit and x4 damage if you attack an unsuspecting enemy
Poison Use: safely handle and apply poisons

Races

Human: two high attributes
Dwarf: resistant to poison, stonecunning talent
Elf: resistant to enchantment, detect secret doors and magic talent
Halfling: resistant to fear spells, once per adventure reroll a failed defensive roll
Drow: see in normal or magical darkness, once per adventure you can blanket a room in shadows (other drow can see through this)
Orc: once per adventure when you hit do double damage

To-Hit Rolls

Roll 1d20, beat the number for your class/level on the chart. High Strength gives +1 on Melee (hand to hand weapons). High Dexterity gives +1 to Missile (ranged seapons).
This roll assumes a creature with moderate armor on. Attacking a low armor creature gives you a +2 bonus, attacking a high armor creature gives you a -2 penalty.

Weapons

Small Weapons do 1d6 damage. Ex. dagger, shortsword, mace, handaxe. Clerics, Wizards, and Thieves.
Medium Weapons do 1d8 damage. Ex. longsword, battleaxe, flail. Fighters.
Large Weapons do 1d10 damage and negates the reach advantage of large creatures like giants, but you cannot use a shield while wielding the weapon. Ex. spear, great axe, claymore. Fighters.

Hit Points

Roll one die per level (varies by class). Add +1 per level for High Con.

Armour Class

Base is 11.
+1 for High Dex
+2 for Leather Armour (Thieves)
+4 for Chainmail (Clerics and Fighters)
+2 for Shield (Clerics and Fighters, cannot be used with Large Weapons)

Saving Throws

Roll 4+ on D6 to avoid danger. +1 bonus for High Wis. Other High Attributes may give bonuses for specific types of dangers.

Spells

I used the spell list from Microlite74 Basic, one of several D&D retroclones. Use your own spell list from any edition of D&D and adapt on the fly, or make your own.

That's it! There are lots of holes that require heavy DM adjudication, but honestly that's in the spirit of Basic D&D.