So basically, I want to create a d20 (or less if I get less submissions) table to determine what kind of cult players can encounter and I was hoping that the community would be able to help me out with this. All I'm asking is for people to submit their ideas for cults, which can be as descriptive as you guys want to make it. I was planning on using a d100 to determine the total number of cultists, but if you want to include in your submission to roll a 2d20s for your amount of cultists or total number of cultists is 58 then you're more than welcome to do that. You can say something like 'the cult has three Abominable Yeti leaders' or 'all cultists are body-builders and use owlbear stats' or 'the cult worships an Adult Brass Dragon'. You could even say that the entire cult is comprised of druids between the levels of 1 and 10 or is 50% clerics and 50% wizards. At the end of the day, I want people to have the freedom to create a cult that will be however they want it to be so the players can have the opportunity to encounter a unique cult in their adventure. I'll use this table once it's done, but anyone else is free to use it as well. I'll update this post with submissions as I get them and I just wanted to say thanks to anyone who helps me out.

Thanks to LeviRocks, StylesStriker, Krrn, DxJxC, NaivaraArnuanna, and Googleguy for the submissions so far!

• A cult that worships the Tarrasque. The members are extremely glutenous and hope to one day be consumed by it in order to become one with it. Food shortages are often seen in areas around cultist-occupied areas.
• A cult that is dedicated to the expand the Mind Flayer colony through willing sacrifices of people or creatures. These members allow themselves to be transformed by intellect devourers (total = 2d20) and work with Doppelgangers (total = 1d20) in order to lure more people in. There are 1d4 subterranean churches that belong to this cult, each housing an elder brain that demands a sacrifice once every 1d12 months in order to please it in the belief that it will reward them and create more Mind Flayers or intellect devourers. Sacrificial rooms are circular, containing stone tables used for sacrifices. These churches also have chambers built for former or current cult leaders, which are often where resources are stored.
• A cult comprised of only Bards, of whatever races the DM chooses to use. The leader is a level 1d20 Bard, with all other members being whatever levels the DM chooses so long as none of them are a higher level than the leader. This cult does not worship any god, instead choosing to worship the power of words. Their persuasive abilities, verbosity, vocabulary, and knowledge of legalese has allowed them to gain full control and ownership of everything and everyone within a village/town/city. They seek out discourse and debate, attempting to gain more members and control through the power of their voice.
• The Cult of the Dragon. This cult worships Tiamat (or a similar dragon, up to the DM) and wishes to bring about draconic control. Roll 1d100 to determine the percentage of members who are Kobolds. They believe that through the help and cooperation of Chromatic Dragons (whether these dragons actually help or acknowledge the cult is up to the DM's discretion) their members can become drakes and half-dragons, since they already have 2d20 drakes and 1d20 half-dragons. Through the use of draconic rituals and necromancy, they have managed to create 1d4 dracoliches that they struggle to control.
• A cult that worships the Elder Elemental Eye, made up of four separate elemental cults that have come together for the 'greater good'. Break the total number of cultists up however you wish, so long as each of the four have members. Each cult worships a different elemental lord; Fire worships Imix, Water worships Olhydra, Earth worships Ogrémoch, and Air worships Yan-C-Bin. 1d20 members of the Fire cult are Paladins that use flaming weapons and explode upon death. 1d20 members of the Water cult are druids that can Wild Shape into watery snakes, have +1 AC from barnacles on their skin and can cast water and ice spells (number and spells are DM discretion). 1d20 members of the Earth cult are proficient in mounted combat atop a bulette (each has one) and can cast Petrifying Gaze once every long rest. All members of the Air cult has a permanent Fly cast on them and used ranged weapons. They have constructed 1d20 mechachimeras that cast Gust of Wind in place of their fire breath. These four cults all secretly think their elemental lord is better than the others.
• A cult that worships a long-dead Necromancer that originally worshiped Orcus and was believed to become a Demigod. The name of the Necromancer has been lost through time (DM's choice) and his description changes depending on who you ask. Some believe s/he was a mass of writhing worms and maggots, others believe he was giant put together by the bones and corpses of many. You have the final choice on what the Necromancer's appearance. The leaders of the cult are 1d4 Star Spawn Larva Mages who all have different opinions on the true appearance of their Demigod. The cult is comprised of School of Necromancy Wizards, 1d20 necromancers, Evil Mages, and Cultists. Their goal is to resurrect the Necromancer and allow the dead to take over.
• A cult comprised of Paladins who have sworn allegiance to a mad Angel who has convinced them that they must cleanse the world of evil. They believe that anyone who comes in contact with magic items can only achieve salvation by means of death, as such they swear to never use any. The leader of the cult is a 1d20 level Paladin (DM's discretion on build) while all other members are of varying levels below that. There are 1d20 Blackguards among their ranks.
• A cult made up of Sailors, Fishermen, Pirates, and Sea Merchants. One thing they share in common is that they all witnessed some sort of horror they've seen beneath the sea and wish to keep it from waking. Once every 1d12 months, they gather at a port city and kidnap a victim to take out to sea for a sacrifice, not knowing that the more they sacrifice the closer they get to waking it. Within their group is 1d10 zealots who want to actually awaken this submerged horror by sacrificing more victims and will try to convince other members of their cult. They have one warship and two sailing ships that they only use for these rituals.
• A cult that is made up of any variety of races and classes that are possessed by the desire to dance and with no true leader (you can make one if you want). They're not necessarily evil, but are being controlled by a Dazzling Orb that constantly emits an aura that compels people with a 60ft radius to dance. Unless one willingly joins the cult by sharing some of their blood with the Orb, you can not resist the Aura and will have to pass a DC 15 WIS save every round. The Orb must roll a d20 if it is attacked and can be destroyed if all the gems are removed (each gem is AC 10) or if it rolls a 1 when it is attacked. Upon destruction, the curse is lifted and a Blood Witch is released who is angry that she has lost control of her subjects.

The first result should be related to the Tarrasque, either worshipping the Tarrasque, or a cult full of Tarrasques.

I must suggest that another cult be dedicated to the Mind Flayer colony expansion through willing sacrifices; people or creatures that knowingly turns themselves over to be transformed by Intellect Devourer, spending time willingly with Doppelganger so they can use their visage to lure more people in, or even outright sacrificial ceremonies that probably don't do anything but the belief that an Elder Brain will reward the cult for following its directives.

Subterranean chambers with Mosques for former cult leaders (who could be pretty much anyone but likely humanoid), and circular rooms with multiple stone tables where the sacrifices lay, awaiting their turn at becoming part of the hive.=D

Also, some Elder Evils cults, Atropus the world born dead, Kyuss the worm that walks, Borem the lake of boiling mud, all of which that have Star Spawn members, and warlock members.

I appreciate the ideas so far, but could you be a little more detailed with your submissions, Levi? I'm not 100% familiarized with lore so I'd be taking a lot of liberties with things and don't want to kind of go off track with your submissions so they're still 'yours' and not mine. Also I hope I did your submission justice, Styles. If I messed anything up, let me know and I can fix it right up!

A cult led exclusively by bards. They are not devoted to any god / gods. Instead, they are devoted to the spoken word and the power of words (i.e. song,  speach, rhetoric, etc.) Essentially, a debate club taken to the extreme.

Looks alright to me. I like handing out little snippets that turn into works of art!

Quote from HenriBecquerel >>

I appreciate the ideas so far, but could you be a little more detailed with your submissions, Levi? I'm not 100% familiarized with lore so I'd be taking a lot of liberties with things and don't want to kind of go off track with your submissions so they're still 'yours' and not mine. Also I hope I did your submission justice, Styles. If I messed anything up, let me know and I can fix it right up!

 Great old ones, basically cthulhu cults.

I also recommend the Cult of the Dragon, Tiamat worshipers who cooperate with chromatic dragons and kobolds, perform rituals, create dracoliches, drakes, and half-dragons.

And a cult of the Elder Elemental Eye (Tharizdun), it's basically a bunch of elemental cults wrapped into one. They worship different evil elemental lords, such as Imix the prince of fire, Ollhydra, the water queen, Ogremoch, the earth lord, and Yan-C-Bin, the air master. Fire cultists can explode upon death, graft metal to their skin, and have flaming weapons and fire resistance/immunity. Water cultists grow barnacles on their skin, control water, turn into watery snakes, and can freeze people. Earth cultists can mold earth with magic, phase through stone and dirt, ride on bulettes, and turn people to stone. Air cultists can fly or glide, make magic darts that seek out targets, create flying fortresses, and push others around with air.

Quote from HenriBecquerel >>

I appreciate the ideas so far, but could you be a little more detailed with your submissions, Levi? I'm not 100% familiarized with lore so I'd be taking a lot of liberties with things and don't want to kind of go off track with your submissions so they're still 'yours' and not mine. Also I hope I did your submission justice, Styles. If I messed anything up, let me know and I can fix it right up!

 Yeah, sorry about that.

Atropus the World Born Dead is a far realm entity (basically an alien) that is a patron for undying lord and great old one warlocks. Refer to this for more information, Atropus the World Born Dead. Atropus's followers are necromancers, cultists, warlocks, and other crazy humanoids.

Kyuss the Worm that Walks was a necromancer priest of Orcus that eventually gained enough followers to become a demigod. His followers are Spawn of Kyuss, necromancers, mad wizards, cultists, and Star Spawn Larva Mages. You can find more information about Kyuss here Kyuss the Worm that Walks.

Borem the Lake of Boiling Mud is a dead Primordial that was killed by The Dead Three gods, nearly 2000 years ago in Forgotten Realms history. His cultists would be trying to resurrect him, and could likely still gain powers from his lingering primordial spirit. His followers would likely be mad transmuters, crazed oracles that foresaw his return, mud-based elementals, and elemental cultists.

Elder Evils are powerful primordial beings that don't exist on the material plane, either in one of the Outer Planes, the Far Realm, or are dead, or imprisoned in some demiplane. They normally can't come to the Material Plane because they're too powerful and bound to their world, or they can't find a way to the Material Plane, or they don't want to leave their current home plane. They can still grant spells and powers to their cultists, clerics, and warlocks, but they don't have power to warp the material plane in their current form and plane.

 You can also use:

• Cultists seeking immortality through lichdom (possibly worshipers or followers of orcus).
• While not truly a cult, hag covens could make a good alternative (they can disguise themselves as humanoids to draw less attention).
• Very typical devil worshiping cult
• The Xanathar guild isn't a cult, but is a criminal organisation run by a beholder, could be a pleasant change of pace after a bunch of cults.

Thanks for all the submissions so far, everyone. For those who submitted multiple ones, I'll hold off on adding them for the time being. I figure 2 max per person is fair and if I hit over 20, then I can freely add anything else to make this a dXX table instead of a d20 one. Hope you guys are okay with my interpretations of things.

Nice ideas. I'll have more ideas if you need more in the future.

You're more than welcome to post your ideas whenever you want. If this gets up to 20 submissions or if I struggle to get up to 20 submissions, I'll add them all in eventually.

Here are a few more ideas (I've made all of these up, no d&d canon supports these):

•  A group of Sun Soul monks and conquest paladins that worship a sun deity named Alathare, and kidnap random citizens that their deity 
• A cult filled with gish NPC's (spell slinging weapon wielders if you're not familiar with the term). Some notable members would be elven bladesingers, githyanki eldritch knights, and hexblade warlocks. These all worship a malevolent force named the Proclaimer, whose doctrine establishes physical and mental might as equal powers.
• Fallen angel cult led by a radiant idol (from ERflW), the fallen angel is trying to gain enough followers to become a god-like power once more. Members could be priests and war priests, celestial warlocks, zealot barbarians, and tribal warriors.
• The cults of the Dead Three, these three "gods" live on the world with mortal avatars that if killed destroy their celestial nature. The gods are Myrkul, the king of the dead, Bane, the lord of tyranny, and Bhaal, the master of murder. Myrkulites weild flails with heads that look like human skulls and contain human souls, Myrkul's followers are necromancers that seek to become a lich, old people who don't want to die, and undead. Bane's followers are oathbreaker and conquest paladins, fighters, and all that seek to conquer others, his followers typically have a blood-soaked red gauntlet on their right arms. Bhaalites are normally assassin rogues, gloomstalker rangers, and all those who use piercing weapons and seek to kill people when they're most vulnerable. The worshipers of Bhaal have the ability to give nearby creatures vulnerability to piercing damage.
• A cult of a lich, who has undead servants and other necromancers and wizards that wish to learn the secret to lichdom from their lichlord. 

A cult of inquisitors (think medieval Catholicism) is systematically executing "impure" humans on the orders of a mad angel.

A cult of sailors, pirates, and fishermen gather every moon to cast a living sacrifice into the sea, a ritual to STOP the waking of the horror they've seen beneath.

a cult of disco dudes of all races and classes who just dance. if you don’t dance with them then they make you or kill you whichever you prefer 

Thanks for all the submissions so far. Almost have enough for a d10 table. Still looking for any others if people want to submit them.

Maybe a cult of people that ride on walruses and torn people into them.

Kicks in door did someone say cult ideas, and do I see a lack of devil cult ideas 

well then I’ve got the perfect thing and it’s different that most cults, the knights of Zariel 

the knights of Zariel are a cult dedicated to the first lord of the nine hells, and they operate more like an order of mercenary knights that try to twist any system or deal into there favor. The knights are close like a band of brothers who have willing sold there souls to Zariel in hopes of becoming a soldier on the front lines of the blood wars for one reason or another, some warriors are manipulated into believing the true glory, and others may have joined because the knights offer them a purpose. They can also gain souls by exploiting nations with less coin into hiring them with souls. The knights are made up of Knights, Cambions, champions, and some other such warriors, also feel free to make a cambion warlord (just mesh a Zariel Cambion and a warlord) or anything or anyone else willing to sell there soul to knights