I am running a holiday game using B2 The Keep on the Borderlands. The Cave of the Unknown has been many things in that module for many people. For this game my Cave of the Unknown is based around +Dyson Logos recent map of The Cave of Mushrooms.
I've stocked it using Labyrinth Lord rules but it should be compatible or easily converted to any old-school system or retro-clone.
PDF and VTT ready .PNG are included as usual.
P.S. Folks playing in my B2 game this season, DON'T LOOK!!
General: The entire cave complex is wet and humid. Water drips from most exposed surfaces and there are many puddles on the ground. Thousands of slugs can be seen feeding on the giant mushrooms. They range from normal size to over 2' in length. Some can grow much larger.
1. Albino Ape* (1) 
2. Carcass Scavenger* (1d2) 
3. Fly, Giant Carnivorous (1d4)
4. Mushroom Men* (1d3) 
5. Lizard, Giant Gecko* (1d6) 
6. Locust, Subterranean (1d10)
*There are a limited number of these creatures in The Cave. If encountered as a wandering monster remove them from their normal lair areas (number in brackets).
1. Main Entrance. The opening from the wilderness is large, however, not much light is able to penetrate the cave from outside due to the depression and tree cover nearby. A warm, humid breeze blows from inside the cave regularly. It carries a musty, loamy smell.
1a. Hidden Entrance. This cave entrance is set low to the ground and is partially obscured by vegetation. Only a focused search in this area will reveal the ingress.
2. Piercers. Deadly Piercers (AC 3, HD 1, hp 4, #AT 1, Dam 1-6, Save F1, ML 10, XP 10) have staked out these areas and will attempt to drop on any warm-blooded creatures that pass below. Roll 2d4 to determine the number of piercers hiding in the ceiling when the characters pass through.
3. Gecko Lair. 6 Giant Gecko (AC 5, HD 3+1, hp 13 each, #AT 1, Dam 1-8, Save F2, ML 7, XP 100) make their lair here. They subsist primarily on the Subterranean Locusts that roam the cave. If the characters disturb a flock of the Locusts anywhere in the cave there is a 50% chance that 1d6 of the Gecko will come to investigate.
The door in the northeast is made of heavy wood, banded with iron, that has somehow not decayed in the many years that it has been here. The magical runes carved into the iron may have something to do with that. The door can only be opened by means of a knock spell or other similar magic. Inside the room is a small table with various alchemical apparatus covered in cobwebs. They could be sold for 400 GP. Painted on the floor is a circle of arcane power. Any magic-user who sleeps fully within the circle for an entire night may memorize the spell locate object for the following day, in addition to his normal allotment of spells, whether he normally knows the spell or not. If the magic-user is of insufficient level to cast locate object he is still aware that if he could cast that level spell he would be able to after sleeping within this circle.
4. Forgotten Entrance. The ledge and steps in this room are not natural formations and have obviously been worked by some creatures. The passage to the northeast eventually leads to the Boulder Filled Passage at area 51 of The Caves of Chaos.
5. Albino Ape Lair. A large Albino Ape (AC 5, HD 5, hp 27, #AT 2 (claw, claw), Dam 1d4+1/1d4+1, Save F3, ML 7, XP 120) lairs high up on this platform. He has a penchant for shiny things and has collected a pile of such. There are 600 GP in loose, assorted coins, a mithril chain shirt +1, and a brass door-knocker with the word Welcome in Bugbear etched into it.
6. Feeding Flies. 6 Giant Carnivorous Flies (AC 6, HD 2, hp 7 each, #AT 1, Dam 1-6, Save F1, ML 8, XP 29) are feeding on the corpse of a particularly large slug. They prefer live meat...
7. Carcass Scavengers. 2 Carcass Scavengers (AC 7, HD 3+1, hp 20 each, #AT 8 (stingers), Dam Paralysis, Save F2, ML 9, XP 135) lair here when they are not out scavenging for carcasses. 1,000 SP inexplicable litter the floor in this area.
8. Murky Pond. The waters of this pond are still and dark. A Giant Crayfish (AC 4, HD 4+4, hp 28, #AT 2 (claws), Dam 2d6/2d6, Save F4, ML 9, XP 140) is often lurking just under the surface, waiting to dart out (surprise 1-3 on d6) and grab a meal. The pond empties into a larger body of water on a lower level via many twisting and turning water-filled tunnels where only a Giant Crayfish can go.
On a ledge about 4' under the surface of the water lies a helmet of the cunning linguist that belonged to a former snack of the Giant Crayfish. The helmet is rust free and detects as magical. It will grant its wearer the ability to speak and understand 3 random languages per day. The languages are re-rolled each morning. If the wearer already speaks a randomly determined language he gains the ability to use that language telepathically with any creature who also speaks that language to a range of 100'.
8a. Sinkhole. This dark hole in a niche in the cave wall descends further into the depths. A hot, fetid wind blows sporadically from below.
9. Guard Post. 4 Mushroom Men (AC 8, HD 2, hp 10, #AT 2 (fist, spores), Dam 1d4, Save F2, ML 7, XP 35) stand guard on this platform. If a Mushroom Man hits successfully with a fist attack, the target must make a Save vs. Paralysis or take a -1 to-hit and damage for 1d6 rounds due to a cloud of choking spores that billow forth. The penalties are not cumulative from multiple strikes but the total rounds affected are.
9a. Alarm! A Shrieker (AC 7, HD 3, #AT shriek, Dam shriek, Save F1, ML 12, XP 65) that resembles the rest of the mushrooms in the cavern is planted next to this door. It will sound its alarm if anyone other than The Mildew Mage or a Mushroom Man approaches within 20' of the door. The alarm is so loud there is a 50% chance that any aggressive monster left alive in The Cave of the Unknown will be drawn to the plateau.
10. Mushroom Man Lair. 8 Mushroom Men (see 9 above) live here. They will move to reinforce areas 9 or 11 in 1d4 rounds if sounds of combat are heard.
10a. The Mushroom Circle. The Mushroom Men in The Cave of the Unknown are actually one large fungal growth and this room is where the nerve stem lives. It is denoted by a ring of small, neon-swirling mushrooms. If this ring is completely destroyed by fire (10 points of damage) all of the remaining Mushroom Men in the cave will die. The Mushroom Men make regular offerings of any pretty stones that they find to their fungal mother. 6 gems worth 500, 150, 60, 50, 20 and 5 GP respectively are lying in the middle of the circle.
11. The Mildew Mage. Long ago a half-mad wizard (AC 9, HD 4, hp 13, #AT 1 or spell, Dam 1-4, Save MU 4, ML 11, XP 300) entered The Cave of the Unknown. He reacted to the strange energies of the area and began to grow toadstools out of his ears and neck. He flourished amid the fungus and finished the journey to full-mad. He holds court over his mushroom subjects from this room. He has the following spells prepares; sleep, charm person, detect invisible and levitate.
4 Mushroom Men (see 9 above) are always in this room and act as bodyguards for the wizard. If things go badly, the Mildew Mage will use his levitate spell to reach the ledge in the northeast corner 30' above the cavern floor. A passage there winds back to the surface and emerges under the roots of a massive tree.
11a. Sleeping Chamber. The Mildew Mage accumulated some very nice furniture over the years. Most of it is thick with various fungal growths, however. There is a silver platter worth 200 GP that is salvageable, as well as a pair of gold candlesticks worth 350 GP. A teak chests that is locked holds the spellbook of the wizard as well as a sack with 100 PP and a ruby necklace worth 500 GP.
11b. Privy. Mushroom Men may not have to use the facilities but the Mildew Mage does. Here is where he does it. Anyone foolish enough to root around in the filth of this room has a 65% chance to be attacked by Rot Grubs (AC 9, HD 1 hit point, #AT special, Dam special, Save 0 level human, ML N/A, XP 5).