|Paul Carrick is likely the only person who|
has illustrated these obscure creatures.
"They were worms about as long as a man and as thick as a man's thigh, cylindrical and untapering. From end to end, as many as a centipede's legs, were pairs of tiny wings, translucent like a fly's, which vibrated unceasingly, producing an unforgettably sinister low-pitched hum. They had no eyes - their heads were one circular mouth lined with rows of triangular teeth each like a shark's." - Fritz Leiber, "The Terror from the Depths"
These creatures are essentially a form of psychic parasites, imbued with a powerful form of telepathy and an insatiable desire for Mythos-related knowledge. They invade dreams, leech off the subconscious mind, and congregate in great numbers in cave-systems connected to Mythos locations; they can be found beneath R'lyeh itself, or Dunwich, or Innsmouth. This makes them a convenient entity for the Keeper looking to catch jaded players off-guard, and can even serve as a weird variation on the "canary in the coal mine" - encountering Tunnelers can be a sign that worse things are afoot in the region. Normally I'm disinclined to include multiple monsters in a scenario; I really dislike "Monster Mashes" in Call of Cthulhu as a matter of personal preference unless they are reasonably connected - such as a Serpent Person using zombies as guards, for example. So we may come back to the idea of Tunnelers as "canaries," or we may not.
Physically, Tunnelers are not outrageous by Cthulhu standards; they've got a good chunk of hit-points apiece (averaging 16-17) and take half-damage from non-magical attacks, as well as regenerating two hit points per round. So, like many creatures, you're not going to want to go head to head against one unless you're very well-prepared; spells like "Shriveling" and "Bless Blade" are going to be your friends here, and chances are if you are encountering Tunnelers, you've likely got a good bit of Cthulhu Mythos knowledge under your belt already - and if not, you will.
The dangerous thing about Tunnelers is that they radiate Mythos knowledge; spending time in a region infested with these creatures can lead to massive sanity losses and an involuntary gain in Mythos knowledge as things you never wanted to know seep into your brain. It begins slowly, 1d4 SAN points lost per week, but it ramps up until you're losing 1d6 points of SAN per day. That's brutal!
Likewise, every Tunneler knows 2-8 spells, which will likely run the gamut of the Grimoire. Besides making the far more dangerous than their physical stats alone would, this also presents the possibility of a deranged sorcerer seeking them out, offering to hunt down the knowledge they seek in exchange for specific spells he desires.
Let's get those updated 7th edition stats up before we go any further:
STR: (3d6+6)x5 = 82
CON: (3d6+12)x5 = 112
SIZ: (3d6)x5 = 52
INT: (3d6+6)x5 = 82
POW: (6d6+12)x5 = 165
DEX: (2d6)x5 = 35
Move: 4/2 burrowing
Av. Damage Bonus: +1d4
Attacks: one bite per round
Fighting 80%, dmg 2d6+DB
Armor: suffers only half damage from normal, non-magical weapons and attacks, round down any fraction; regenerates 2 hit points per round after being wounded, but dies immediately upon reaching zero hit points.
Spells: 2d4 of the Keeper's choosing.
Sanity Loss: 1d3/1d10 to see a Tunneler Below.
So what do we do with these creatures to make them the basis of a scenario? I touched briefly on using them as a sign of darker things afoot - in which case I would probably have them drawn to "big" castings like Gate Spells and summonings like a moth to flame; imagine them as the remoras attached to the underside of a Hunting Horror or Servitor of the Outer Gods. I would have them especially attracted to things that weaken the barriers between dimensions; Sentinel Hill in Dunwich I would honeycomb with their tunnels in response to summonings of Yog-Sothoth there over the years, for example. Likewise if I were to run the Shadows of Yog-Sothoth campaign again, I would make them a hazard in the "Look to the Future" chapter. I might treat them as creatures of another world that were accidentally brought into ours through injudicious spellcasting; Lovecraft's "From Beyond" might not be a bad place to look for ideas about using these monsters as well (and the gelatinous "Terrors from Beyond" will be appearing here one of these weeks as well), replacing the Tillinghast Resonator with a Time Gate or the like.
But to make them the focal point of a scenario? That's a bit trickier. I think the way to go about it would be to use them at the midpoint, or a little later, in an ongoing campaign. Once the Investigators have begun gaining the knowledge they need to combat the Mythos, they've also, paradoxically, gained the knowledge they need to be appealing to the Tunnelers Below. In a campaign like Shadows of Yog-Sothoth, I'd use them once the Investigators have become a serious thorn in the human villain's scheme - the sorcerer reaches out to the Tunnelers and offers them some big, juicy bit of Mythos knowledge (which he may or may not have) in exchange for the destruction of the Investigators. Then the Tunnelers are off to collapse the Investigators' house into a giant sinkhole or drive them crazy.
In fact, I think it's even more interesting if the sorcerer doesn't have the knowledge he's bartering with, and is bluffing the Tunnelers; maybe he's found a way to protect himself from their psychic probings. This gives the Investigators, maybe, the ability to bargain with the Tunnelers and maybe turn them back on the sorcerer. These are not unintelligent parasites, after all; their average intelligence is that of a very smart human.
For that matter, they would make an interesting alternative to Nyarlathotep for a more Mephistophelean take on the Mythos; present them as tempters and makers of bargains, offering power to the powerless, revenge to those who seek it, maybe even presenting their relationship with their victims not as parasitism but as symbiosis. "You scratch my back, and I'll scratch yours," as it were.
Let's run with that for a minute, shall we? What if we declare the Tunnelers drawn, not to "Mythos energy" or "Mythos knowledge," but to human suffering - which is, perhaps, the same thing. They would be found, then, in places of human atrocity, feeding on the misery and despair and feeding back even more of the same into the area, creating an intensifying loop of suffering. Here and there, they find "points of darkness" - human beings who rage instead of despair, human beings who seek to strike back at any cost. These, the Tunnelers make contact with.
This, I'm realizing more and more, is the key to the horror of the Mythos; it's not the monsters or the tentacles, its the depths of cruelty and ignorance humans will gleefully plunge into; the Mythos is at its most frightening when it's a tool in the hands of an all-too-human figure with recognizable, even sympathetic, goals. Use the Tunnelers to facilitate that plunge. Whisper the name of Azathoth in the ear of a child soldier in Africa. Offer to undermine the homes of Wall Street's elite. The rewards reaped never justify the price one eventually pays.
The price may seem innocuous - to borrow an idea from Leiber's fictitious magic system of Megapolisomancy, maybe all the Tunnelers want are certain signs chalked or painted in a certain pattern of specific buildings throughout the city - individually, they are nothing, just a few lines. But in conjunction with all the other signs, they could open a massive rift into the realms of Yog-Sothoth.