The other day I was reading through the Advanced Dungeons & Dragons 1st edition Monster Manual & came upon the gelatinous cube entry. Every single dungeon that I've played in has had a gelatinous cube or some variety of that monster within it. Why?!
Every dungeon or wizard's tower seems to have one to clean up the remains of adventurers. They are hyper efficient at what they do. They sweep up corpses & remains then spit out the treasures within to their 'masters'.
But where do the damn things come from? Well imagine having a demonic spy in your dungeon & residence listening in on every conversation, seeing everything happening, & privy to all your arcane secrets! Because that's exactly what almost every wizard, lich,red dragon tyrant who owns a gelatinous cube or one of the myriad of variations has in their basement!
Why because a gelatinous cube is the easiest thing that a black necromancer or wizard can summon. Hell, its almost but not quite a cantrip but a first level powerful spell.
The caster uses a simple summon circle to the Faceless Lord Juiblex & another Lovecraftian deity. Other entities must be appropriate to the campaign setting & within 1d4 rounds a miniature gelatinous cube forms within the circle. The cube doesn't have a mind per say but will seemingly follow the simplistic commands of its master the summoner. The summoner will detect no thoughts, ideas, or any other mental activity isn't going to show up. The cube is more of a storage medium & records everything that's around it. Later a small portion of the monster will return to its demonic or Lovecraftian masters to report all that its seen, touched,etc.
Lovecraftian gods of this type most often are from the Advanced Deities & Demigods by Ward & Kuntz especially Shub Niggurath . The Faceless Lord Juiblex & Shub Niggurath are responsible for birthing some of the most henious oozes, horrors, & jelly monsters across the planes when the summoner gets an unexpected & deadly result of their spell!
If the DM decides that a new monster is needed for an adventure then the caster gets far more then they bargained for. Black puddings, red oozes, stun jellies, even the occasional proto shoggoth has resulted from proper & botched rites.
Stun Jelly Fiend Folio
Treasures are looked over & evaluated in the dungeon, capabilities are accessed, & threats are measured on the planes prime across the multiverse. Every couple of 1d6 years a gelatinous cube will do an informational dump back to its masters! Beware the wrath of the Faceless Lord & its ilk my friends. Few wizards seem to realize the grave danger that they place themselves in when summoning these creatures!
Summon Gelatinous Cube
1st level Wizard Spell
Components -chalk, candles, small animal, the gloves of power
The caster creates a small 3 foot diameter circle of power in full moonlight using the forbidden & hidden names of The Faceless Lord Juiblex & Shub Niggurath. Within 1d4 rounds a gelatinous cube will begin to form and coeless into a familiar cube shape.
The caster will have had to prepare an enchanted pair gloves of rubber ahead of time for 1 gold piece each. These will allow the summoner to move the little proto monster as needed
It is known as Juiblex, also called The Faceless Lord,
Lord of Sloth, Demon Lord of Poison and Ooze. It is the demon lord & father of Slimes and Oozes but its other names are Szhublox & the faceless rot. Juiblex's lair is on the 222nd layer of the Abyss, which he shares with Zuggtmoy, the Demoness Lady of Fungi. Together they are fungal rot & chaotic evil incarnate tearing apart the soul of the planes.
Juiblex appeared in the Advanced Dungeons & Dragons Monster Manual in 1977. In first edition, Juiblex rules his own layer of the Abyss, the 528th layer, which is an infinite sea of slimes and oozes feeding off each other's putrid forms.
Juiblex is one of the most dangerous of the Abyss's demon princes not because of its ambitions but because its already among us. Slimes, jellies, & puddings are a part of him & his children. They are everywhere spreading from dungeon to dungeon moving among the planes like a fungal infection. He rules not only his own plane but the Undersump as well. A series of planar tunnels that are networked throughout the Abyss connected by the river Styx. Not only does he know all but is aware of everything going on around his children. Green slimes infect a myriad of dungeons & even though they are not intelligent in the traditional human sense. They relay a trillion bits of data back to their demonic parent whose vary existence is a mockery to all of creation. 
Slimes move into dungeons & ruins across the multiverse giving Juiblex access to an infinite number of planes. The power of the Grey Oozes are not to be discounted. His silent assassins & spies stalk the halls of power & have claimed many a life of a powerful wizard or even some powerful demi god children. These silent assassins are often used by his worshipers such as powerful Drown families, drug users, & poisoners of divinity along with royalty. 
The powerful bruisers of the dungeons are the Ochre jellies who are spawned by the cult rituals & rites of the cults of Juiblex. They are the strong arm of these cults taking on any who challenge the right of them to worship their parents inside dungeons & ruins. They are the hand of the entropic demon lord ruining the lives of any who challenge their lord's worship. 
The power of ochre jellies should never be underestimated & it is said that their demonic master himself uses these bastard horrors as his tools on the prime material. These 'blessed' jellies often seem to exhibit a remarkable degree of cunning & evil intentions. They can use tactics & traps to murder any adventurers they come across as sacrifices to their 'father'. Since the planes are in the "Kali Yuga (Sanskrit: कलियुग, translit. kaliyuga, lit. 'age of Kali') is the last of the four stages (or ages or yugas) the world goes through as part of a 'cycle of yugas' (i.e. Mahayuga) described in the Sanskrit scriptures.[1] The other ages are called Satya Yuga, Treta Yuga, and Dvapara Yuga.
Kali Yuga is associated with the demon Kali (not to be confused with the goddess Kālī). The "Kali" of Kali Yuga means "strife", "discord", "quarrel" or "contention".
Juiblex's power & influence has been on the rise as is the cycle of Abyss moves into descendancy;"Undoubtedly, few denizens in the Abyss equal, in the horror they evoke or in the hideousness of their forms, the corrupt majesty of the Faceless Lord. In the gloom that enshrouds his realm, he holds court over the glistening masses, the twitching oozes he sires. Juiblex is deemed a lesser figure in abyssal hierarchy, judged Lord of Nothing by his peers. However, in my research, I have plumbed the foul mire to examine the full darkness that is this disturbing fiend. Juiblex is a power defined by incremental advances, a being whose wickedness flows slowly but inexorably toward an unspeakable end, which troubles even this writer."
I've always loved the Demon: Juiblex artwork by christopherburdett
Juiblex is the premiere alien demon lord whose long standing form of chaos incarnate has been the subject of being one of the possible sources for the creation by the Elder Things of the naught impossible horrors of the shoggoths. Because of the mating of Juiblex & Ubbo-Sathla ("The Unbegotten Source", "The Demiurge") according to The Mysteries of Science which gave rise to the most dangerous & demonic of these foul monsters.
The Pit of Shoggoths by Azaghal
Juiblex's influence should never be uncounted. He or it is one of the most dangerous of demon lords. Ever present & waiting to corrupt & strike. His reign of terror starts when & where its needed coming into full bloom when adventurers least expect it. Here are ten ways I've modified Juiblex to fit in my home campaigns:
- I've used his other areas of worship to great effect for his dungeons & temples besides the usual fungal aspect of the demon lord. These include Chaos, Destruction, Evil, & Water.
- Juiblex's human cults are very dangerous & think that destruction of everyone & everything is good for the multiverse. They will murder when & where it suits them.
- Juiblex has a strong on going rivalry with Tsathoggua over certain regions of the underworld & upon Hyperborea.
- There are write up on Juiblex in the Wraith of the Immortals set for his place on Mystara
- Cults of Juiblex often pay for rare & exotic molds, slimes, & jellies
- The temple areas & ruins of Juiblex are often infected with some the most dire of chaotic diseases & infections
- His influence pops up in the post apocalyptic wastelands as a god from time to time among wasteland nihilists
- Gamma World has three temples dedicated to the demon god.
- Juiblex has been summoned to destroy worlds in a post apocalyptic fungus fueled frenzy over millions of years.
- Juiblex is actually one of a race of powerful demon lords & he is their father.
