I might be DMing a 5e game; plans so far

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(Planning on running this for my coworkers, probably; here's my first draft.  First post will be the stuff I give to the players; next post will be some NPCs and stuff)

The setting is as follows: a standard D&D world with magic, D&D races, etc.  Much of the planet worships The Great Cthulhu, many indirectly through its herald, the Silver Shepherd.

Clerics of the Silver Shepherd
Clerics of the Silver Shepherd wish to feed and foster life on the planet, bring about world peace, and preserve as much life as possible.  This is the largest "Lawful Good" group.

Falken Necromancers
This chaotic evil faction opposes law and order, and claim obvious heresy such as "Cthulhu is a tentacle monster that wants to eat this world!  The Clerics of the Silver Shepherd are fattening you all up!"

Druids (and other religions)
Druids have long memories.  They remember a time when Cthulhu was not worshiped on this planet, and are naturally mistrustful of those who claim Cthulhu is the "one true god."  But they are at peace with the Clerics of the Silver Shepherd who seem to preserve life and the forest.  (As are many other older religions).

Warlocks of Cthulhu
This group is seen as a bit old-fashioned.  Historians claim they were the ones who brought the good word of Cthulhu to this planet.  But Cthulhu is so wise and so great that communing with it directly seems to prove...perilous for mortal minds.  Most prefer to worship the Silver Shepherd.

Paladins of the Silver Shepherd
"Power corrupts.  Absolute power corrupts absolutely."  Therefore: the prime directive of these paladins is to slay the most powerful to ensure peace.  They hunt down the Falken Necromancers, all varieties of dragons, Warlords who seek to conquer, etc.

The secular populace
Bards, Rogues, shopkeepers, town guards, Barbarians, and Sorcerers, oh my.  Much of the population cares little for religion, but benefits from the peace and food it brings.  A few people would like more power for themselves, but fear (and resent) the Paladins of the Silver Shepherd who would interfere.

---

Party alignment--unlike most D&D campaigns where running with an evil-aligned party members is off-limits, evil alignment might be an option if, for example, the party decides that Cthulhu and its followers must be destroyed at all costs.  (This would open up NPC classes such as Oathbreaker Paladin and Death Cleric).  What will be more important is party consensus--there will probably come a decision point in the campaign, where the party will choose between law and chaos: try to pick characters who could be swayed to either side.

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NPCs and NPC classes

Clerics of the Silver Shepherd
These are Life clerics with a slightly modified spell list to focus more on food and agriculture--the always prepared spells:

Level 1
Goodberry
Purify Food and Drink

Level 3
Lesser Restoration
Prayer of Healing

Level 5
Plant Growth
Revivify

Level 7
Death Ward
Control Water

Level 9
Mass Cure Wounds
Raise Dead

In addition, Clerics of the Silver Shepherd have the Eldritch Blast cantrip instead of the Sacred Flame cantrip.  (They don't, however, have any of the Warlock invocations--that would require multiclassing).

These will almost all be Lawful Good.

Warlocks of Cthulu
Just standard Great Old One warlocks, with the one caveat that they will almost all be Lawful, but can vary on alignment from good to evil.

Paladins of the Silver Shepherd
Just standard Vengeance Paladins (they already seem well-suited to david and goliath type encounters).  Although again, they will generally be Lawful.


NPCs

The Silver Shepherd
She is an angel (let's say a Solar, but add in the Eldritch Blast cantrip with all the invocations).  She is in fact, Lawful, but NOT Good.  Not evil either--she is a technically a thrall obeying her master.  Speech-wise she acts like Sailor Moon ("in the name of Cthulhu I shall punish you!")

Lesser Shepherds
Although not worshiped like the Silver Shepherd, there are multiple other shepherds (angels), taken pretty much from the Angel section of the DMG, with some Eldritch Blast mixed in.

Wednesday Falken
One of the Falken Necromancers.  Character speech/behaviour/appearance inspired by Wednesday Adams.  Rumors of her being a Lich are sadly untrue, although she keeps meaning to try (she just hasn't had the time, unfortunately).  She knows that Cthulhu is a world-eating monster, which she finds rather admirable of it.  Still, though, she'd really rather not have to planeshift her entire mansion to another dimension, so all things being equal she'd prefer it if Cthulhu didn't consume this particular planet.  Level 20 Necromancy wizard.

The Great Cthulhu
Admitedly, not a fight the players are supposed to win.

I'm using Tarrasque as a baseline for stats (Except with int 30 20 instead of int 3) mixing in a bit of Kraken and Mind Flayer and Great Old One warlock (with Int instead of Cha as spellcasting modifier).

AC: 25
Hit Points: 676
Str 30, Dex 11, Con 30, Int 20, Wis 11, Cha 11
Saving Throws Int +14, Wis +9, Cha +9
Damage immunities: Fire, Psychic; Slashing, Bludgeoning, and Piercing from nonmagical sources
Condition immunities: Charmed, frightened, paralyzed, poisoned
Blindsight 120 feet.

Legendary Resistance 6/day (yeah, no, not leaving this at 3/day for a fight that's supposed to end in "you all get eaten"--too much risk of something like Quivering Palm)

Magic Resistance (advantage on saving throws against magic and spell like effects)

Reflective Carapace: Any time Cthulhu is targeted with Magic Missile, a line spell, or a spell that requires a ranged attack roll, roll a d6.  On a 1-5, Cthulhu is unaffected.  On a 6, Cthulhu is unaffected, and the spell reflects back on the caster.

Spellcasting: Cthulu is a Wizard with save DC of...27? (8+9(prof)+10(int)) mmm...ok, this is probably too high of a DC.  I'm just going to fudge this down to DC 22 (same DC as a level 20 Warlock with Rod of the Pact Keeper +3).  Actually...I could just make Cthulu's int 20, which gets the same effect, and makes its int save not the comically high +19.
(1) Dissonant Whispers, Tasha's Hideous Laughter
(2) Detect Thoughts, Phantasmal Force, Misty Step
(3) Clairvoiance, Sending
(4) Dominate Beast, Evard's Black Tentacles
(5) Dominate Person, Telekinesis
(6) Mass Suggestion
(7) Plane Shift
(8) Telepathy
(9) Gate

In addition, Cthulhu can communicate telepathically, and can turn an incapacitated creature into its thrall.  It also has the Mind Flayer's Mind Blast--cone 60 ft, DC 22 Int save for 4d8+10 psychic damage, and stunned (save again at the end of each turn).

Actions:

Multiattack: Cthulhu uses its controlling presence, and then makes 5 attacks (two with its claws, and three with its tentacles).

Controlling Presence: DC17 wisdom save--every creature within 120 feet must make the save or be charmed for one minute.  Creatures can repeat the saving throw every turn, but with disadvantage if Cthulhu is within line of sight.  Once they succeed on this save, they are immune for 24 hours.

Claw: +19 to hit, reach 15 ft, 32 (4d10 + 10) slashing damage

Tentacles: +19 to hit, reach 30 ft, 24 (4d6 + 10) bludgeoning damage, and the target is grappled (escape DC 20).  Until the grapple ends the target is restrained.

Fling: instead of making a tentacle attack, Cthulhu can fling an already grappled target up to 60 feet.  If the target strikes a solid surface, they take 4 (1d8) bludgeoning damage for every 10 feet thrown.  If the target was thrown at another creature, that creature must succeed a DC 20 Dexterity saving throw, or take the same amount of damage and be knocked prone.

Legendary Actions
3x per round

Swallow (costs 2 actions): eat a large or smaller creature that is grappled by Cthulhu.  While swallowed: blinded, restrained, total cover against everything outside of Cthulhu, take 56 (16d6) acid damage at the start of each of Cthulhu's turns.  If Cthulhu takes 60 damage or more in a single turn from a creature inside of it, it must succeed a DC 20 constitution saving throw at the end of that turn or regurgitate all swallowed creatures.

Tentacles: make a tentacle attack or a fling

Move: move up to half Cthulhu's movement

[Dark Holy Elf]

While it's cool to stat out lategame optional boss ideas, don't get too attached to them right now; you really don't know what the party will look like at a level that will fight them yet and that can make a big difference to balance. Just means you'll have to revisit this encounter with a discerning eye much later, if necessary. (Quite apart from balance reasons, you'll probably have a better idea by then of what works and what doesn't from a "fun for the players" perspective.)

unlike most D&D campaigns where running with an evil-aligned party members is off-limits

Hahaha, good one.

[Captain K]

Using Cthulhu specifically by name will give your players preconceived notions.  Call it something else so the players don't know that the Silver Shepherds are the bad guys.

(non- A)D&D had an arc like this in the Hollow World setting.  Thanatos the evil god created a false religion pretending to be a good god.  A minor immortal saw through this and picked a PC to be their representative.  The PCs had to expose the false religion by getting the actual good immortals to take notice of what was actually happening.

[metroid composite]

Using Cthulhu specifically by name will give your players preconceived notions.  Call it something else so the players don't know that the Silver Shepherds are the bad guys.

(non- A)D&D had an arc like this in the Hollow World setting.  Thanatos the evil god created a false religion pretending to be a good god.  A minor immortal saw through this and picked a PC to be their representative.  The PCs had to expose the false religion by getting the actual good immortals to take notice of what was actually happening.

Well...that's fair...although I've already run the concept past some of the players, so it won't be a surprise to half the party if I change it now.  (At least one of them has stated that he'd be down with fattening up the populace for The Great Cthulhu, so it's entirely possible the party will be working with the forces of Cthulhu anyway).

Also, it opens up the realm of "Hail Hydra" jokes with NPCs running around saying "Hail Cthulhu".

[Grefter]

The Silver Shepherd
She is an angel (let's say a Solar, but add in the Eldritch Blast cantrip with all the invocations).  She is in fact, Lawful, but NOT Good.  Not evil either--she is a technically a thrall obeying her master.  Speech-wise she acts like Sailor Moon ("in the name of Cthulhu I shall punish you!")

Be still my beating heart.

[metroid composite]

unlike most D&D campaigns where running with an evil-aligned party members is off-limits

Hahaha, good one.

Well...most campaigns I've been in with my coworkers. >_>

While it's cool to stat out lategame optional boss ideas, don't get too attached to them right now; you really don't know what the party will look like at a level that will fight them yet and that can make a big difference to balance. Just means you'll have to revisit this encounter with a discerning eye much later, if necessary. (Quite apart from balance reasons, you'll probably have a better idea by then of what works and what doesn't from a "fun for the players" perspective.)

This sounds like good advice, yeah.

(The only reason I listed out some numbers for Cthulhu is that generally Cthulhu arriving on your world is supposed to basically instantly end the world.  Like IIRC in Arkham Horror you don't even get to fight him, the game just ends.  There are other solid options like "don't let Cthulhu get summoned", or alternatively "cheer alongside other loyalists as Cthulhu gets summoned, and be happy that you will be the first ones eaten.")