A while ago, I worked on Caverns of Slime for Fight On! #14. I recently made it available as a free PDF. (I should add a license to it: Creative Commons Attribution Share Alike for the text!) Here are some of its monsters I wrote up.
This list also contains two instances of rituals. I describe them as follows: “Rituals are the ceremonies and prayers that take many minutes to perform, usually powered by demon lords and devils.” The first monster can be summoned by a ritual that player characters can be taught by “a winged pig man with four legs and wielding a blood falchion.” The last monster, on the other hand, performs a handful of rituals that player characters can learn through observation.
Bel, Slayer of Men, a summoned demon: HD 10; AC 2; Atk 1 flaming whip (no damage but on a hit it grants +4 on the hit with the sword in the same round and on a 20 it disarms the opponent), 1 flaming sword (3d6); MV 9; flaming aura deals an extra 1d6 to everybody nearby; immune to non-magic weapons; immune to fire); this may be the same ritual that the mage-wraith in the Warrens of the Troglodytes had learned
Merquishti, a giant intelligent spider hungry for meat – and conversation: HD 8; AC 4; Atk 1 bite (1d6); MV 15; when bitten, save vs. poison or fall comatose for 24h; can set up area web traps); it speaks the language of giant spiders, a dialect of subsurface elves and the common tongue of all demons; if asked about gold, the spider will know that a knight’s golden armor was carried down this way by devil worshipers a “very long time” ago; if asked about the destination, the spider will mention a city of evil half-spiders preying on the caverns and tunnels below; when the conversation ends, the spider considers capturing a party member by triggering a web trap (save vs. wands or be trapped in a web; the web prevents spellcasting; trapped victims need to deal 20 points of damage using a dagger or a similar small blade; free allies can aid trapped victims using larger blades
Earth Elementals surface, eager to take back any gems the party may have “stolen” from the deep earth: HD 6; AC 0; Atk 1 bite (2d6); MV 9; immune against fire and electricity; can melt into the surrounding rocks in one round
Nixies enjoying a bath, apparently ignorant of any males approaching: HD 3; AC 8; Atk 1 bite (1d6); charm man 3×/day, kiss of water breathing for charmed men at will
Stone Polyps are growing on the stone walls, immobile, with their arms limp and soft; as soon as a warm creature passes nearby, however, they activate: HD 6; AC 0; Atk 1 bite (2d6), 8 tentacles (save vs. petrification of fall unconscious and be dragged towards the mouth); MV 3
Shroom Lord is growing his own circle of psychotic colors: HD 15; AC 5; Atk 1 smash (2d6), 1 lick (poison); MV 9; licked victims must save vs. poison or be charmed
The shroom lord can be observed for days as it sings its telepathic praise to the Gibbering Fungus God. A very patient person interested in mushroom magic can learn the following rituals through simple observation:
Another monster, related to the circle of psychotic colors: any enemies that fell asleep because of the spores will die over night and rise as a spore zombie.
Spore Zombie: HD 2; AC 8; Atk 1 bite (1d6); when smashed, anybody in melee must roll a d6 as described above