Fraz-Urb’luu is the smartest creature alive. He sees the nature of all things and manipulates them to his own end, disguising himself and his intentions with myriad layers of lies, tricks, and promises. His cult is larger than it appears because it mostly comprises people who don’t know they’re working for him, and his favorite trick is convincing somebody that they’re part of the forces of good when they’re really furthering his plots.
From his description in the books, it seems like he’s surprisingly active, making personal appearances whenever he feels it appropriate. He doesn’t sit back and let his followers work for him as much as he instigates situations and lets them play out exactly as he expected. It’s entirely possible for a party to meet him very early in a campaign and not realize it until much later, when it’s almost too late to stop him.
Typical Followers: Fraz-Urb’luu’s typical follower honors him voluntarily. In that way anybody can be his follower, which isn’t especially helpful advice. It’s more interesting to focus on other liars and manipulators who use his power for themselves, from illusionists to criminals to overly ambitious politicians. Among monsters, look to rakshasas, who disguise themselves as powerful humanoids, and slaadi, who delight in causing chaos. It’s not enough to just be able to change shape or lie; a true follower of Fraz-Urb’luu actively works against the structures of law and good, and any such actions can fall in line with his plans.
Atypical Followers: Any creature could secretly be a follower of Fraz’Urb’luu—that’s the point. His worshippers disguise themselves as something else, pretending to be ordinary paladins or barbarians or shopkeepers so they can work out of the spotlight. Typically “smart” creatures like mind flayers or beholders make sense, but at lower levels you have more room to work with creatures gifted in deception. Faerie dragons are smarter than the average person and can turn invisible at will, traits that also fit fey like pixies and sprites. These specifically work better against intelligent PCs, who know these creatures are normally good-aligned. It makes their inevitable betrayal all the more surprising.
Plots: The best plots for Fraz-Urb’luu look like they don’t involve him at all. They seem innocuous at first and only reveal themselves when the players are too deep to ignore it, like convincing them to destroy a demonic cult among high society so he can fill the power vacuum with his thralls. Any prophecy you use could be planted by him, and any ally could be one of his agents in disguise. This also means any of the PCs’ allies could be subject to his machinations, and anybody from their patron to their friendly rival could need their help escaping him.
The danger here is using too many layers of deception. Don’t move evidence of Fraz-Urb’luu’s influence so far behind the scenes that players never find it, and don’t have them fall into his traps time and time again. It befits his strategies, but it’s not a good play experience.
From the Vault: It’s hard to find something from previous editions about deception because it’s not a well-covered rules space. Player’s Handbooks explain what the Deception or Bluff skill is and provide some example DCs, but they mostly leave adjudication to the DM after that. There’s also no such thing as rules for creatures who are good at planning. The closest thing I can think of is a bit of loose session design. DMs, statistically, are rarely as smart as a lich or other high-Intelligence creature, and it’s reasonable that the creature would think of things the DM didn’t. Instead of defining all of the creature’s abilities, leave one or two of them blank. For example, decide several of a lich’s spells but not all of them. When a situation arises in which the lich would need a spell it doesn’t have, give it that spell, because the lich thought of a contingency you didn’t. Again, use this sparingly, but it’s a great “just as planned!” moment.
We don’t need to pull any illusion spells or powers from previous editions; the 5E options do a good job of it. Instead we can do something better—instead of pretending to be something else, a follower of Fraz-Urb’luu can actually be something else. The chameleon is a prestige class from the 3.5E Races of Destiny. Originally intended for humans and doppelgängers exclusively, it lets a character switch out certain character traits every day, like feats, class features, and ability scores. It even lets a character decide whether or not they have spellcasting on a given day. Applying this to a creature involves some on-the-fly math, but it lets the same creature perform multiple functions at different times. All of the options below can scale with level, and you can add other options to suit your needs:
Chameleon (1/day). This creature selects one of the following options. This option gives the creature a bonus for 24 hours or until it uses this ability again:
Arcane Focus: This creature’s Intelligence increases by 2 and it gains proficiency in Arcana and Intelligence saving throws. This creature is a 5th-level spellcaster. Its spellcasting ability is Intelligence and it selects from the wizard spell list.
Bardic Focus: This creature’s Charisma increases by 2 and it gains proficiency in Deception and Charisma saving throws. This creature is a 5th-level spellcaster. Its spellcasting ability is Charisma and it selects from the bard spell list.
Combat Focus: This creature’s Strength increases by 2 and it gains proficiency in Athletics, Strength saving throws, and martial weapons. A melee weapon deals one extra die of its damage when the creature hits with it.
Divine Focus: This creature’s Wisdom increases by 2 and it gains proficiency in Medicine and Wisdom saving throws. This creature is a 5th-level spellcaster. Its spellcasting ability is Wisdom and it selects from the cleric spell list.
Resistance Focus: This creature’s Constitution increases by 2 and it gains proficiency in Constitution saving throws. This creature has advantage on saving throws against spells and magical effects.
Stealth Focus: This creature’s Dexterity increases by 2 and it gains proficiency in Stealth and Dexterity saving throws. Once per turn, this creature deals an extra 7 (2d6) damage when it hits a target with a weapon attack and has advantage on the attack roll, or when the creature is within 5 feet of an ally of this creature that isn’t incapacitated and this creature doesn’t have disadvantage on the attack roll.
Wild Focus: This creature’s Wisdom increases by 2 and it gains proficiency in Perception and Wisdom saving throws. This creature is a 5th-level spellcaster. Its spellcasting ability is Wisdom and it selects from the druid spell list.