Adapting Secrets of the Nethercity—a sprawling, "classic-style" underworld crawl—to Adventurer Conqueror King System II (ACKS II) is a match made in gaming heaven. Both share a love for high-lethality exploration and meticulous underworld ecology.
Since Secrets of the Nethercity was designed for a similar OSR chassis, the conversion is less about rewriting the book and more about mapping the specific "levers" that ACKS II provides for dungeon delving.
1. The Core Conversion: Monsters and NPCs
Most monsters in the module can be used as-is, but ACKS II introduces more nuance in how it handles Hit Dice (HD) and Special Abilities.
Attack Throws: Calculate the Attack Throw based on the monster's HD using the ACKS II monster progression (typically starting at 10+ for 1 HD).
Saving Throws: Use the ACKS II categories: Blast, Breath, Death, Gaze, Staff, and Wand.
Special Abilities: Check the ACKS II Monstrous Manual. If a Nethercity creature has a unique "gimmick," map it to an existing ACKS proficiency or monstrous trait. For example, any necrotic aura should likely require a Save vs. Death.
2. Treasure and Economy
The "Gold to XP" ratio is central to both systems, but ACKS II has a very specific Monthly Revenue and Market Class logic.
The GP/XP Ratio: ACKS II generally assumes $1 \text{ gp} = 1 \text{ XP}$. If the treasure hauls in the module seem too high for your desired leveling speed, scale them down by 20–30%.
Appraisal: Use the ACKS II Appraisal proficiency rules for the various statues and relics found in the Nethercity. This adds a layer of tension—do the players risk carrying a heavy statue that might be worthless lead?
3. Navigation and Lighting
ACKS II treats the "Dungeon Move" as a primary resource.
Mapping & Movement: Ensure you are tracking movement in 10-minute turns.
Light Sources: ACKS II is stricter about light. The deep, airless vaults of the Nethercity should emphasize the weight of oil and torches. If the party runs out of light, use the Mishaps and Calamities table.
Dungeon Procedures: Use the ACKS II Dungeon Exploration flow:
Wandering Monster Check (usually 1-in-6 every 2 turns).
Check for Exhaustion (every 5 turns of movement).
Check for Light/Resource depletion.
4. Proficiency Checks
Secrets of the Nethercity often asks for "Ability Checks" or "X-in-6" chances. In ACKS II, these should be converted to Proficiency Throws.
Searching: Use the Sensing or Finding Hidden Doors mechanics (usually 18+ on 1d20, modified by the Mapping or Dungeon Engineering proficiencies).
Trap Disarming: Map all mechanical traps to the Traps proficiency. If a character lacks it, they generally cannot attempt a disarm beyond simple "prodding with a 10-foot pole."
5. The "Conqueror" Transition
The Nethercity is unique because it isn't just a hole in the ground; it’s a subterranean urban environment. As players hit higher levels (Level 5+), lean into the Domain Management rules of ACKS II.
Clearing Levels: Once a level is "cleared," players might want to establish a Sanctum or a Hideout within the ruins.
Henchmen: The Nethercity is deadly. Encourage players to use the ACKS II Mercenary and Specialist rules to hire torchbearers and man-at-arms. Managing a 20-person expedition into the deep city is very "on-brand" for ACKS.
Summary Table for Quick Conversion
| Original Element | ACKS II Equivalent |
| Armor Class (AC) | Descending to Ascending (or leave as-is if using ACKS II descending options). |
| Saving Throws | Map to the 5/6 ACKS II categories. |
| Skill Checks | Proficiency Throw (usually 18+ or 14+ with proficiency). |
| Encumbrance | Use the Stone system (essential for the heavy loot in the city). |
| Morale | 2d6 check (crucial for the many factions in the Nethercity). |
Adapting Secrets of the Nethercity to Burrows & Borderlands (B&B) shifts the tone from a traditional "human-centric" dungeon crawl to a high-stakes, small-scale epic where the environment itself is a colossal antagonist. This blog entry picks up from OSR Commentary - AX3 Capital of the Borderlands (revised edition) By Alexander Macris, & Newton Grant For Adventurer, Conqueror, King II
In B&B, your players are brave rodents and small mammals. The sprawling "Nethercity" isn't just an ancient ruin; it’s a terrifying, cavernous underworld where a simple cracked pipe is a waterfall and a stray housecat is a legendary dragon.
1. Scaling the Environment
The most immediate change is Scale. To a mouse or shrew, the vaulted ceilings of the Nethercity feel like the sky.
The "Giant" Problem: The "giants" who built the city are humans. Most of the original inhabitants of the Nethercity (if they were humanoids) should be treated as Colossi.
Verticality: B&B emphasizes climbing and gliding. Use the rubble and "giant" furniture of the Nethercity as 3D platforming challenges. A staircase isn't just a path; it’s a series of treacherous cliffs requiring Athletics or Agility checks.
Hazards: Standard dungeon traps (darts, pits) become much more lethal at a small scale. A 1-inch deep puddle is a pond; a drafty hallway is a gale-force wind.
2. Creature Conversions
You’ll need to re-skin the Nethercity’s bestiary to fit the "Animal Adventurer" theme.
| Original Monster | B&B Equivalent | Notes |
| Goblins/Kobolds | Rats or Weasels | The primary rival factions. Weasels are more elite/predatory. |
| Zombies/Skeletons | Blighted Insects | Use the "Blight" or "Infection" themes. Undead mice are tragic; giant, rotting beetles are terrifying. |
| Ghouls | Starved Ferrets | Fast, relentless, and unnervingly flexible. |
| The "Boss" Monsters | Stray Cats or Owls | These are Tier 4 threats. A single owl in a high-domed chamber is a campaign-ending boss. |
3. Loot and Scavenging
In B&B, gold coins are heavy, impractical, and often useless. The "Gold to XP" mechanic should be replaced with Scrap and Provisions.
Valuable Items: Instead of jewelry, players find Shiny Bits (buttons, needles, glass shards). A rusted sewing needle is the equivalent of a +1 Longsword.
Provisions: High-quality food (dried berries, a hunk of "giant" cheese) is more valuable than gold.
Armor: Instead of Plate Mail, players find Thimble Helms or Soda-Tab Bracers.
4. Navigational Hazards: The "Darkness"
B&B characters often rely on scent and whiskers, but the Nethercity is unnaturally deep.
The Scent-Map: Allow players with high Cunning or specific animal traits to "track" scents through the dungeon. However, the "Ancient Rot" of the Nethercity can overwhelm these senses, acting as a "Scent-Obscuring Fog."
Light: Torches are massive logs to a mouse. Players will likely rely on Glow-worms, bioluminescent fungus, or "borrowed" matchsticks.
5. Faction Maneuvering
The Nethercity is famous for its competing factions. In B&B, these should be handled via the Disposition system.
The Rat King: A bloated, stationary ruler in the lower levels who controls the flow of "Giant-waste" (garbage).
The Bat Scouts: A group of high-altitude observers who offer "Air Support" (information) for a high price in shiny objects.
The Insect Hive: A mindless, territorial force that acts as a natural barrier between the "Upper" and "Lower" Nethercity.
6. Mechanical Tweak: Stress and Terror
Because the Nethercity is so alien to surface-dwelling forest creatures, use a Stress or Terror track.
Claustrophobia: Spending too long in the "Stone Tunnels" (which don't smell like earth or grass) imposes a penalty to Spirit or Willpower saves.
The "Great Silence": The lack of wind and forest noise in the deep Nethercity can be unnerving. Force a check when the party enters a particularly dead/quiet zone.
