FOMORIANS (the Fhoi Myore from the Chronicles of Corum)

They look like this, more or less.

 Armour Class +7     Hit Dice 13     Move 75% Normal Human     Morale 10

Seven rotten mountains stumbled out of the sea or from under the ground, groaning under the weight of their diseases, and found the world not to their liking. Too loud, too warm, too bright - it is agony to them. They seek to make conditions more hospitable by laying the peace of death on the mortal sphere, muffling it in eternal snow and endless fog.

Gigantic and at least roughly humanoid, the gross and unpleasing shapes of the Fomorians are mercifully obscured from mortal sight by a dense shroud of freezing mist. Yet their hazy bulk can still strike terror into those that glimpse them.

They creak across the landscape in hefty chariots and carts - slowly, inevitably, like entropy - never setting foot on the mortal earth. It might be that they deem themselves too grand to walk or have lost their divine toes to millennia of metaphysical gangrene.

As well as bringing the Hounds, the Fomorians are served by the Half-Dead and Bitter Dryads. They are also served by mortals and others that give up on resisting them in the hope of surviving just a little longer, or because they too yearn for oblivion.

They seem capable only of making hooting, braying, squealing, snorting and gibbering noises, without language or wit. Or maybe this is the sound of unseen beasts that pull the carts, and the Fomorians are no more the masters here than the Bitter Dryads?

The Fomorians are gods:

• Of a sort. Not omniscient or omnipotent, but can be assumed to have plot armour and legendary actions as needed.
• Their effect on the local environment (fog and winter) expands and becomes self-sustaining the longer they remain, eventually changing the climate indefinitely.
• Use subvocal/telepathic communication without barrier of language or distance. They know things. Can send visions. Prefer to deal via intermediaries (Bitter Dryads). Will not acknowledge direct communication in the field.
• Their divine presence causes awe and horror:
• 1st to 3rd level suffer -2 to all dice rolls except damage; 4th to 7th level -1. Higher levels no penalty, but can feel it.
• For NPCs, monsters, animals, also apply to Loyalty/Morale.
• At the start of a combat round, a Fomorian can force a Loyalty/Morale check on individual NPC of max. 3rd level as a free action (or whatever you want to call it) 
• Minimum 6 hp/HD and use d6 up to d12 (depending on system/edition/needs).
• Invulnerable Monsters and min. possible damage from non-magical source.
• Regenerate damage from any source, 1 hp/round, even from 0 hp or less.
• Very hot/large (min. 10+ hp damage per attack/round) and magical fires cause half damage and the Fomorian must take a Morale test or retreat 1 combat round.

Their presence changes local conditions:

• Fog surrounds them out to normal/unadjusted wilderness encounter ranges.
• Gives disadvantage on all vision, distance, getting lost and missile attack rolls.
• Fomorians, Hounds and mounted Bitter Dryads are unaffected.
• The Fomorians and their allies get advantage on surprising you (or a +1 to +3 situational bonus), or advantage on stealth etc. Encounter range is point-blank.
• Aura of cold causes cumulative harm in 1 round increments at melee range.
• Outside of encounter range, use longer intervals as appropriate.

They can make one attack per round for 4 damage dice:

• 4d6 or 4d8 depending on game/edition.
• Describe/narrate as appropriate: uprooted tree as a club, giant rotting fist, filthy serrated bronze knife, boulder hurled from out of the mist, a ponderous cart rolling inexorably over as they ignore you in the fog.
• Wounds at least double chance of infection with revolting and unusual diseases.
• vs. monsters/NPCs only: excess damage from kills cleaves others nearby until used up.
• Compare damage roll vs. Ability Scores and AC (either ascending, or 20-minus-descending, or something) for special weapon effects (excess as hp damage):
• net of intestines and spinal columns: vs. STR or pinned (d6 targets in a group).
• whip of tangled ancient roots: vs. DEX or grappled at range (min. Fomorian STR is 19).
• bolas of severed heads: vs. DEX or pinned; vs. STR or KO'd.
• ballista-sized crossbow bolt: vs. CON or impaled and pinned until you can remove it (STR check per round, excess on failure is more damage).

One of them has a baleful Evil Eye:

• It's Balahr/Balor, and that Eye can be difficult/slow to open.
  
• Gaze automatically ignites combustible material. This is normal fire.
• Save or spontaneously combust for 2-13 unsoakable magical fire damage. 
• Save or be turned to stone.
• Fail both and turn to stone, then collapse/explode into smoking chunks - irrevocably dead, even if you're otherwise immune to magical fire.
• Range as you like, AoE as you like, non-flammable cover must save or collapse/explode.

One of them carries the Horn of the Hounds:

• In the Chronicles, this is the chief of the Fhoi Myore, Kerenos. Described as male, with antlers, if that helps. Calatin, a mortal wizard, is able to counterfeit the Horn, so its power does not appear solely concentrated in the object.
• The Horn is at least giant-sized, and requires min. STR 18 to lift/blow. 
• It can be used to command, dismiss, slay and summon Fomorian minions. I suggest only the Hounds can be conjured out of nothing, coalescing out of the mists and bloodstained snow.

They are vulnerable to the Tathlum:

• A missile/sling shot made of brains and bones of your enemies, ideally used against their relatives. They are effective for one attack only, successful or not.
• Process can be as simple or as convoluted as you like, with or without magical ingredients and spells to enchant it. Resists mass production, resulting in numerous unidentifiable duds (and possible curses, if magical).
• Tathlum damage to Fomorians regenerates at equivalent natural healing rate rather than 1hp/round, and is fatal if it brings them to 0 hp or less.
• If made of the remains of Bitter Dryads, Half-Dead and/or Hounds, the Tathlum inflicts normal sling/thrown rock damage. 
• On a crit, all Fomorians present must make a Morale Check.
• If made of the remains of a Fomorian, roll to hit vs. unarmoured AC and it inflicts flat damage equal to half the donor's undamaged hit points. No save and all Fomorians make a Morale Check at ML 8. 
• Instant kill on a crit, and all Fomorians make a Morale Check at ML 6.

Commentary.

I rather like this summary description of them as implacable leprous nihilistic geriatrics with a death wish.

The Fhoi Myore/Fomorians occupy an elevated position in my imagination, hitting the confluence of the Cthulhu Mythos, Steven King's The Mist, the Mists of Ravenloft, mythological retelling and (retrospectively) Silent Hill and Shadow of the Colossus. The Chronicles/Moorcock suggests that they are remnants, the bitter end of the deposed Chaos Gods, thrown out of their Heaven and eking out their existence by spreading their doom around. 

Figures of pity as much as horror, this affected my view of weakly godlike colossal Cthulhuvian entities since. They (and things like them) are uncomfortable and unhappy and without much hope - we'd sympathise if they weren't also actively incompatible with our existence. Or possibly if they were sexier.

The Chronicles is fairly low on detail on the Fhoi Myore, but does give hints and names. Balahr and Kerenos are the most obvious links back to Irish myth.

One (possibly two) are characterised as being female, whatever that would mean in their context - in the Chronicles, it means the sexual mutilation of male victims. Sreng is described as wearing a kilt of seven swords, but not if these are captured mortal swords or of more appropriate scale. One is credited with the creation of the Ghoolegh - is this an exclusive ability or just a statement of innovation?

One is named Bress, and as one of the Irish Fomorians is Bres the Beautiful, I like the idea that he/it can project an appearance more palatable to mortals or even a semi-independent astral body not working entirely in the interests of the original.

The motive force for the carts/chariots is opaque in the Chronicles, but it could be magic or alien technology or some horrible beast of burden (or dominant symbiote, as noted above).

I used the basic Fomorian statblock from AD&D 1e Monster Manual 2 as a mechanical starting point. They might need beefing up depending on your campaign - I'm envisaging a world where Corum is only a 4th level Fighter (a Hero, right?).