It's funny, I don't normally go in for seasonally-themed game content. Halloween gets a pass, though, because creepy stuff and gaming go together like a good ribeye and liquid fucking awesome. In this vein, here's the Nightgaunt rogue archetype: they wear masks and thrive on the terror of their enemies.
...Until their hearts are torn with grief I shall accuse them, til the very Stone shakes and cracks. I shall keep no fetter upon the horrors within the Mask.
I was after making myself a fine new mask. It was perfect, so it was.
Nightgaunt Roguish Archetype
The tradition of the Nightgaunts is little known, and by their very nature, few who encounter them are willing to speak of them. They craft or alter masks, which they wear into battle; the terrors that they bind to these masks gives them great power over the fears of their allies and enemies - even enemies who are hardened against dread.
At its founding, the line of Nightgaunts believed they were bearers of holy dread, reminding people to seek solace from the gods. It was all too easy for corruption to seep into their ranks, as they exulted in the fear and deference of the masses. Nowadays, only a rare few Nightgaunts aspire to any sort of greater cause.
Your spellcasting ability for all Nightgaunt features is Charisma.
The Mask of Horrors
At 3rd level when you choose this Archetype, you craft or alter a mask, according to the secret lore of the Nightgaunts. When you wear your Nightgaunt mask, you gain some control over the ebb and flow of fear. When you or a creature within 30 feet of you makes a saving throw against the frightened condition, you may either add one point to your Terror pool, or spend your reaction to grant advantage or disadvantage on the saving throw. You may store any amount of Terror, but your Terror pool returns to 1 after any short or long rest.
Further, you gain all three of the following abilities:
When you or an ally within 30 feet of you kills an enemy that is immune to the frightened condition, gain 3 points of Terror.
As an action, spend 1 point of Terror to deal 1d8 psychic damage to frightened enemies within 30 feet, as your mask contorts to inspire unspeakable dread in your enemies. For each additional point of Terror that you spend on this feature, increase the damage dealt by 1d8, to a maximum of 5d8.
As an action, spend 2 points of Terror to cast chromatic orb, using your mask in place of its material component. When you use this effect, the eyes of the mask burn with terrible power. Each time you use this feature, its Terror point cost increases by 1 until you complete a short or long rest.
Starting at 3rd level, gain proficiency with the Intimidation skill. If you are already proficient in Intimidation, add it to your list of Expertise skills. If Intimidation is already one of your Expertise skills, you may gain Expertise in a new proficient skill.
As an action, you may cause either all enemies or all creatures within 30 feet to make a Wisdom saving throw. Creatures that fail this saving throw are frightened until the end of your next turn. Once you use this feature, you may not do so again until you roll initiative, or complete a short or long rest.
The Arts of Night
Starting at 9th level, you inscribe additional sigils on the inside of your mask, granting it further powers. Choose three of the following; once you choose them, you may change them whenever you gain a rogue level.
Spend 1 Terror point to add 15 feet to your reach until the end of your current turn. All weapon damage you deal this round is changed to psychic damage. You may not use this feature more than once per round.
Spend 1 Terror point to prevent a target within 30 feet from making opportunity attacks until the end of your next turn. You may not use this feature more than once per round.
Spend 1 Terror point to gain darkvision until you complete a short or long rest.
As a reaction, spend 1 Terror point to make a melee weapon attack and move up to your speed whenever an enemy provokes an opportunity attack from you.
As an action, spend 3 Terror points to become invisible to all frightened targets. This invisibility lasts for one minute, or until you make an attack or cast a spell. For purposes of the frightened effect, you are in their line of sight no matter where they go until you end the invisibility effect, but their movement is not constrained.
As an action, spend 3 Terror points to cast hold person on one target that you have frightened, without spending a spell slot.
As a bonus action, spend 2 Terror points to teleport a distance up to twice your Speed. The destination of this teleport must be adjacent to a frightened creature.
As an action, spend 6 Terror points to cast phantasmal killer without spending a spell slot.
As an action, spend 3 Terror to cast confusion on a single creature within 30 feet.
When you deal Sneak Attack damage to a frightened creature, gain 2 Terror points.
As a bonus action, grant an ally a new saving throw against the frightened condition. As usual, you may either gain 1 Terror point from this, or give your ally advantage on the saving throw.
Starting at 13th level, you may cast dream without spending a spell slot. When you choose to make the messenger appear monstrous as you cast this spell, you gain 2 Terror points. A second casting of this spell before completing a short or long rest costs 3 points of Terror, a third casting costs 6 points of Terror, and so on.
Starting at 17th level, you can sense whether a creature within 30 feet is immune to the frightened condition. As a bonus action, spend 1 Terror point to force a creature within 30 feet of you to roll a Wisdom saving throw or lose its immunity to the frightened condition for 1 hour or until you fall unconscious, or you are on a different plane of existence. If you or an ally within 30 feet kills it while it is denied this immunity, you still gain 3 Terror points.
There are three major inspirations for this class - Freddy Krueger, DC's Scarecrow, and the Dust to Dust character known as the Tendril of Ruination. So, arguably, this archetype calls you to play the anti-hero, if not an outright villain, but I figure that 5e is all about that (warlocks, y'all), and this is Halloween. Incidentally, Nightgaunts are also a creature of Lovecraft's Mythos, which Wikipedia describes thus:
Nightgaunts have a vaguely human shape, but are thin, black, and faceless. Their skin is slick and rubbery. They sport a pair of inward-facing horns on their heads, and have clawed hands and a long barbed tail which is used to "tickle" their victims into submission. They can fly using a set of membranous wings. They make no sound.
So that's fun.
I definitely could have designed this to be simpler. There's a fair amount of data tracking going on, what with the Terror pool. On the other hand, it's not much worse than the Battlemaster, the monk, or the sorcerer, so it shouldn't make anyone's head explode. I feel like the Terror currency captures the sense of ebb and flow as well as any mechanic would.
The thing I care about here is that it's a subclass that lives and dies by the frightened condition, but is still fun to play and has options when facing something immune to the frightened condition. It drives me absolutely batty that the Archfey warlock hangs so heavily on charmed and frightened, but gains no new tools for dealing with enemies immune to those effects... and a lot of creatures are immune to those effects.