If you have not read Black Christmas the Campaign this submission does not make as much sense out of context. So go read that. Don’t worry, I’ll wait. ..*tuneless whistling* You are back? Great. Now read on.
The Grinches are extradimensional creatures that come to the world via Wreaths (Magical Gateways in Elf Speak). Grinches are not a born race, but seem to be made by the malevolent intelligences on the other side of The Wreaths. Made out of some form of psuedo-matter, akin to fey matter, the monsters manifest as they reach the material world. They come in a variety of forms, but they are all identifiable by some constant elements.
They are Green and Black in coloration. They all end up being oddly furry. Their skin is short nappy and "felt like". Their eyes come in a variety of looks, but red and glassy seem to be the most common variation. If their actions did not reinforce their name in the first place, the green motif does make them Grinch-like.
The name Grinches comes from The Elfs who came to fight them. The Human Media people who covered the attacks latched on to the name. It has stuck since then.
They are only somewhat intelligent. They are extremely intelligent pack hunters. Sometimes they will snatch pieces of christmas such as hats, garland, bulbs, and tinsel to wear… much the way a "traditional tribesman" might decorate themselves to scare their opponents or steal its power for themselves.
They do communicate primitively through screeches and wails. The sounds also seem to generate fear or nails on chalkboard annoyance in Humans.
Normally they are just some what intelligent monsters. A Grinch will occasionally show great cunning. Perhaps the intelligence behind The Grinches are puppetting those intelligent ones. If one "Smart Grinch" is taken down, another will often take its place. (At any given time, one Grinch in a pack might have this cunning.)
They are probably not normally "material" as we think of it. They come from a "place" that probably has different rules. That is why they are all twisted, mishapen, and odd when they come through. Even versions of the same "type" are not quite the same.
Each pack of Grinches seem to be tailored for each Mission/ Assault. Each "Mission" seems to have an accompaying Wreath in which they come out of. Destroying or closing down The Wreath will prevent reinforcements and weaken the existing Grinches. This explains their variation of shapes and abilities. If they are simple Grinches who use violence, then a good sized pack of 12 Grinches (3-4 per PC on Elf Assault Team) is unleashed through The Wreath. If they are an empowered Grinches, they will often have a small pack of 5-6 Grinches supporting them. ("Ninja" Grinches seem to run solo). Additional Grinches might emerge from The Wreaths, if left undiscovered.
Unfortunately, even after Black Christmas, very little is known of The Grinches.
They are probably multidimensional, which is why they can have the odd making powers. By moving through the sixth and seventh dimensions, they can effect the three dimensions in ways impossible for us.
Time probably does not flow in their home realm as it does here. It is possibly not continuous as we think of it. If it is continuous, it is fast. This is becomes evident in their Christmas motif. They studied our world briefly, then maybe again, then maybe again, then sent them through. Little did they realize that is was 350 to 370 days between peeks. Their quick view of us showed that we have this "faith system" related to Christmas (and they lump Hanukkah and Kwanzaa in as well, and Diwali and possibly even Ramadan, into it as well). This is our belief and they seem to want to destroy it. (If they had viewed more or different times, they would of saw other beliefs in action. Then again, perhaps to their view of things, this is more important than our other beliefs).
What is their purpose in on Earth? Well it does seem that destroying The Core Human Holiday does seem to be "their goal". They are striking at Holiday Events (Carnivals, Christmas in the Park, Concerts, Carolling, etc), and places where people gather (Malls and Airports). They seem to be avoiding churches and such for the most part.
While the monsters do kill, and often do it in interesting ways, they seem more into the hurting, maiming, and chaos. They like the screaming crowd.
This leads us to an alternate option.
They seem to need emotions. They obviously feed off them. And they seem to prefer dark emotions, as they are powerful and easy to produce. So we have pain, despair, and the crushing of hope. This seems to be their bread and butter. If not for the Grinch Minions, at least for their Masters (Some Elfs and Angels have taken to calling them Dark Hearts) on the other side of The Wreaths. This idea is lent credence, as The Grinches do not attack Churches, Temples, etc, as the people in attendance might not fall to despair since they are charged with their faith.
Additional Ideas (5)
These rough mishapen humanoids are the bulk of any Grinch Pack. They are a bit taller than Humans (twice Elf height), broad in shoulder, with oversized hands and feet.
Their ability to smash and crush is great. They are fairly tough.
They occasionally ooze ectoplasm, giving them their snotty, snot rug, and shambler labels.
These creatures resemble flying Manta Rays roughly five feet across. They don't actually flap a wing to fly, but seem to float. As they undulate the move through the air. These mostly black creatures have green octipi tenticles emerging from their bottom sides. These tenticles are not strong, but they do allow the Sky Rays to grab children and drop them from a good height.
These creatures only seem to come out after they have lost an engagement or two to flying opponents.
These small shamblers tend to dress themselves up with ornaments and tinsel. They can animate Christmas Trees (And Garland) into monsters (they grow psuedo feet/ legs and arms with claws). These new monsters are adept at smashing and chasing people. The tree monsters sometimes entangle people with stretched limbs of lights or throw ornament bombs. Taking out the Tree Master restores the trees to normal.
The Grinches are attempting to destroy The Great Season. While it seems they are focused on Christmas, they are attempting to destroy all hope. This creature is a nine limbed monster that can control any fire in a moderate distance. It can control the intensity and size of the fire, as well as shape and other properties.
This can cause a moral dilema for the Elfs. Their mission is to defend Christmas. However the people being attacked are probably not celebrating Christmas. Angels and Demons, as well as many other Mythics/ Figments, will defend them.
(Oddly, if some other force attempts to harm those of the Judaic faith, these monsters will defend them (creating mayham on the other side.)
These look like short humans in ninja costumes, made of ugly green felt. Well a ninja with 1 decimeter (4") claws. The Creeper is all about infiltrating places and attacking from suprise. The Creeper will often kill a few victims early on, so he can chase and scare the others. (Usually a creeper seals his victims into a space, so he can milk their fear and despair without them escaping).