"Call of Cyberthulhu"

an unofficial CoC modification for cyberpunk genre gaming

by Sandy Antunes (sandy@clark.net)
Chaosium's Call of Cthulhu has solid mechanics for interacting with both the mundane and the bizarre. Extrapolating this to the Cyberpunk genre is very straightforward. The main alterations are: addition of new skills and gear (including cybernetics), slight modification of the combat system to intensify the experience, and provision of a "future history" to provide a preliminary setting.


The cyberpunk setting assumes a near-future setting, most appropriate with the Cthulhu Now rules. It is entirely optional whether this setting includes the Cthulhu Mythos at all, as the system is strong enough to work without it. Without Cthulhu, Sanity is still a relevant statistic, as described later.

Augmentation to the human form is available in two formats-- standard Cybernetics, which are machine parts designed to be grafted onto the human body, and Biotech, which are organic devices created to be incorporated into the human body.


The standard CoC statistics all apply. "Education" can be relabeled "Training", to indicate both college learning and learning on the streets. "Sanity", especially for a non-Cthulhu game, may be renamed "Humanity", as it will serve as a measure of how much cybernetics a person may incorporate into themselves.

Adding cybernetic and biotech brings benefits, but also costs in Sanity(/Humanity), indicating that the person has lost a little of their touch with human reality by assimilating technology so much. In a non-Cthulhu setting, this merely indicates one's empathy with human concerns. In a Cthulhu setting, a lowered sanity can be quite significant-- people loaded up with tech tend to fall to pieces quicker when faced with non-technological menaces which defy reason. Sanity(/Humanity) also defines how much hardware a person can add on and still remain "human".


The following skills are useful for a cyberpunk setting:

  • Computer Use (from 'Cthulhu Now') (25%): required for using any computer, as computers are an integral part of society.
  • Computer Tech (0%): This includes both software and hardware creation and modification, for computers and cyberwear.
  • Cooking/Bartending (10%): Bartending is added to the idea of a Cooking skill to reflect the general lifestyle in cyberpunk-- a lot hanging out at bars.
  • Interrogation (0%): Always a useful skill, includes detecting lies.
  • Mechanical/Robotics (20%): This combined skill reflects both general mechanical aptitude and ability to repair or work with the mechanical parts of cybernetics (but not the human part).
  • Pharmacology (0%): Important subdivision of Medical, for the cyberpunk world. Default is 0%.
  • Battlesuit (0%): The ability to use powered armor, exoskeletons, etc.
  • Netrunning (0%): Required for people who will directly link into the information Net (i.e. "Netrunners"). More on this later.
  • BioEngineering (0%): The equivalent of mechanical aptitude for the field of biotech, bioengineering, or genetics.
  • Cybernetics (0%): A mix of mechanical skill and medicine, is required for doctors who actually install or repair cybernetics.
  • Drive CyberRig (10%): A Drive skill, for 'riggers' who cybernetically link to the equipment they operate (often trucks, gun turrets).
  • Heavy Machine Ops (10%): A catch-all for operating non-cybernetic heavy equipment.
  • Tracking (10%): The ability to find or follow someone.

    Brief Technology Skills example

    Jerry the Rigger had his truck lit on fire, his cybernetic arm crushed and torn off, and his biotech eye burned out. Someone with Mech/Robotics could easily fix his truck, and could fix the damage to the arm, but couldn't really reattach it. Someone with Cybernetics could both fix the arm and reattach it, but would have a hard time fixing the truck. Someone with Medical would be able to reattach the arm, but not fix the arm or the truck. None of them could fix the eye, but Medical or Cybernetics could try to reattach it. Biotech could fix the eye and reattach it, but not really do anything for the arm or truck. Simple, yes?

    New Weapons Skills

    Not much is needed in the way of new weapons. "Staff/Stick" (10%) includes most ideas like stunning billy clubs and such. "Blackjack" (25%) refers to use of a blackjack or cosh (for rending people unconscious). "Bite" (20%) becomes useful for people who augment themselves, as does "Claw" (25%). Long range weapons may be added as desired, three to add are: "Tasp" (20%), a weapon which gives a jolt to the pleasure (or, if designed, pain) center of the brain from a distance, "Laser", for any beam weapons, and "Launchers", a catch-all for the rocket/grenade-launcher class of weapons.

    Implanted Options and Cybernetics

    Abilities and gear can be added directly to the character's body by spending skill points, just as with buying skills. More things can be added than I can list, here is a start. Two types are available-- Cybernetics (your standard technology, metal and plastic), and Biotech (more advanced and user- friendly). Biotech costs 50% more than Cybernetics, but is less prone to disruption and outside disturbances. For example, a really big magnet can play havok with a metal/plastic arm, but a biotech arm is safe. Biotech is also more subtle, and less likely to be noticed or detected by equipment. Adding tech reduces one's Sanity(/Humanity), to calculate just divide the number of skill points spend on gear (base cost, not including the 50% addition for Biotech) by 5, then subtract (Biotech costs more to "buy", but doesn't cost more for sanity.) Thus Jerry the Rigger can buy a cyber arm for 20 skill points (costs 4 sanity), or a biotech arm for 30 skills points (still only costs 4 sanity).

    Options below give the cost (for standard Cybernetics) and advantages. Items that give 'special stuff' give obvious benefits which are impossible without them, for example, telescopic eyes would allow one to see very far. Bonuses to "notice" are to the "Spot Hidden" skill.

  • Cybernetic Eyes: cost = 25 pts, adds no bonuses but you can add options
  • Hearing enhancement: cost = 25 pts, +15% to Listen skill, each additional +10pts = +15% to Listen skill.
  • Smell enhancement: cost = 20 pts, +10% to Notice, +10% to Tracking, some poison detection.
  • Enhanced reflexes: cost = 20 pts for each +1 added to Dexterity.
  • Netrunning rig: cost = 30 pts for the basic rig (required to netrun) or 70 pts for the enhanced rig (see Netrunning rules)
  • Socket jack: cost = 10 pts, allows use of 'skill chips' as well as the ability to be linked into machinery.
  • Embedded weapon: cost = 20 pts if you have a cybernetic/biotech limb, or 40 pts otherwise.
  • Cybernetic limbs: arms and hands add to Str or Dex, legs to Con or Dex
  • Netrunning: Only people with "Netrunning" skill and a Netrunning rig can Netrun. To make a Run, they must first connect with the Net. This requires two things-- a place to connect to, and a successful netrunning roll. If they have the basic netrunning rig (above), they must remain at the location where they connected for the Run. The enhanced rig allows encoded remote connecting so they can be mobile (and remain with the party) while doing a run, and it is HIGHLY recommend because 1) it is easier for the player, 2) it is easier for the gamemaster, 3) other NPC netrunners will laugh at anyone too amateur not to have the enhanced rig, hurting your reputation. Netrunning is a whole story in and of itself. To prevent a simplified version of the system I used, there were two types of runs. The rest, like all good cyberpunk games, is Atmosphere and Attitude. One of the best advantages of using the CoC system for CP is that you always have the option of expanding the game into the Mythos, anytime the players start to get too set in their ways... Any questions, feel free to email me at sandy@clark.net.


    Combat is essentially "hit them better than they defend". your "to hit" roll by more than they make their "dodge" roll (if they are able to dodge). For Cyberpunk, this gives a big edge to the faster player, as is appropriate. Also, for combative netrunning, this "do it by more" system applies for the PC netrunner going up against the NPC netrunner.

    Each player is allowed two "actions" per combat round. This is generally assumed to be one attack and one defense (typically "Dodge"). You are allowed to Dodge as many attacks as come at you. Dodging while occupied with another task (driving, wiring up a robot, etc) is at 1/2 your normal Dodge skill. If you wish, you can do a double attack (but not be able to defend), or do a double defense (not attacking, but able to use another weapon to parry or to seek cover-- note that you can't "double Dodge", the extra defense must be something other than Dodge). You can also choose to aim and fire, giving up any defense. Aimed shots, if they hit, hit where you decided.

    You must make your attack roll by better than they make their defense roll for the attack to hit. If it hits, there is a chance it will miss due to cover (see below) or reduced by armor (see section in CoC manual on armor).


    Cover is often a factor in combat. The Keeper will decide the relative value of cover and adjust "hits" accordingly. For non-aimed shots, cover is just a percentage chance of hitting the person (instead of the cover) if the shot hits. Aimed shots never hit cover. People using cover are allowed their normal Dodge (which represents them ducking out of the line of sight), even for aimed shots. Typical values of cover are:

  • Inside a normal apartment, shooter outside across street = 50%
  • Standing behind a corner shooting at someone down an alley = 20%
  • Standing on a roof, shooting down = 40%
  • Standing behind a car = 50%
  • Hiding = generally reduces the chance of being hit by 1/2, for example, 40% cover implies 60% chance of being hit, so hiding increases the cover to 70% (40 + 60/2). This assume you are not fighting but still trying to keep track of the situation. If you want to totally Cower behind cover, you a 1/3 reduced chance of being hit, but will be totally clueless as to the situation (who is there, where they are, what is happening) Most vehicles provide at least some cover in combat. The following are guidelines for dealing with this, when someone is trying to shoot the person (it is easy to hit the vehicles!):

  • Car: 50% chance a non-aimed shot will hit the car, not the person targeted
  • Motorcycle: 15% chance a non-aimed shot will hit the bike, not the person
  • Truck: 60% chance a non-aimed shot will hit the truck, not the person


    A Rigger is a person who hooks themselves directly into a machine to use it. Through a standard Rigger's interface, the person becomes almost one with the machine, increasing their ability and speeding their response time. This mix of person and technology gives the Rigger an edge over the manual operator. The most common use of rigging is Driving-- truck, motorcycle, or pursuit cars in particular benefit from the options and speed of rigging.

    The basic rules of rigging are thus:

  • Rigger items confer extra abilities, for example, radar or early warning,
  • Quality rigger items add to the Rigging skill (typically +5 to +25%),
  • Rigging allows you two attempts to succeed at any task-- first, you roll against natural skill; if this fails, you roll against Rigging to see if the machine/person interlink is able to force a success. For example, a driver with a 75% Drive and 50% Rigging in a rigged vehicle trying to jump Snake River Canyon has to fail a roll of 75% AND a roll of 50% before there are any problems-- so the odds are 80% in her favor. If her bike is quality and adds 25% to her Rigging skill, this means the odds are 85% in her favor,
  • In addition, riggers may use another (single) skill of theirs, while at No Penalty (as long as the skill is physically possible.) This is because the item handles most of the minor details, leaving the rigger free to spend their attention elsewhere. So shooting, first aid, computer library use, photography, and many other skills are possible while rigging. The only universal exception is Netrunning-- due to the brain's bandwidth limitations, you cannot effectively use a Netrunning rig and a Rigger's rig simultaneously without penalty.

    Extended Example

    Jason the Rigger has a Drive skill of 55%, a Rigging skill of 55%, and a dodge skill of 105%. Jason is driving his Rigged motorcycle, a Harley-Davidson 1200 with direct-input controls, CoolLock autosteer, ProxyRite proximity alarms, passive AI control support, submillimeter wave detectors for guidance, UnSeen stealth design, heat signature minimization, AI-controlled weapons support, an autoturret, and armor. The Keeper has rated the bike as follows:

    On this bike, Jason's effective Driving skill is 65%, his Rigging skill is 80%, and his Dodge skill is (105/2)+20, or 72%, not to mention he has a 30% chance that the bike armor absorbs any hits to him. In general, his chance of succeeding in driving/rigging is better than 85%. He has a 70% chance of hitting and gets 3 shots per round using the autoturret. While driving, dodging, and firing the autoturret, he is still allowed one more reasonable action (such as Navigate or Photography or using his shotgun) at normal skill.

    The game system "Call of Cthulhu" and relevant concepts within are trademarks of Chaosium Inc., and no copyright infringement is intended. References to "Call of Cthulhu" game mechanics remain the property of Chaosium Inc., while the setting and background provided herein are concepts by this author and do not represent an official Chaosium product.

    Index | rules | timeline | character sheet | GuideStar | newsblurps | bibliography
    Return to the Gamer's Realm

    Last modified February 3, 1997 by sandy@clark.net