Corruption. Taint. Insanity. Mutation. Warping. The idea that some threats are so horrible, so alien, that dealing with them permanently changes the heroes is very compelling. It’s the cornerstone of Call of Cthulhu games. But whereas in CoC it is an inexorable march toward damnation, it was Heroes of Horror, an excellent D&D 3.5 book, that introduced the rules for taint that really gripped me. In it, taint was horrendous, but also a source of power. Heroes were still sliding towards damnation, but they were damn cool on their way there.

Since reading that book, corruption has been a staple in my D&D games, and the fight against it is one of the foundational concepts of my setting (and my as-yet unfinished novel set in it). Here, then, are my rules for corruption in 13th Age. I tried to capture the playfulness of the system, its unconventional uses of d20. While specific abilities presented utilize 13A concepts, the core mechanics can probably be ported to any other D&D game without any issue. Finally, this version of corruption is written with aberrations as its main source in mind. But it’s trivial to modify or extend it to demons, undead, or some other source of taint, too.

PCs have permanent corruption, which ranges from 0 to 20, and current corruption which starts off equal to the permanent corruption, can never go below it, but can go beyond 20. Unless desired otherwise, new characters start with 0 permanent corruption.

Enemies and other threats that may cause corruption have a corruption rating: d6 for Adventurer-tier sources, d12 for Champion and d20 for Epic; one die for regular monsters, two for double-strength or Large monsters, and three for triple-strength or Huge monsters. Thus a Large Champion-tier monster would have a corruption rating of 2d12.

Whenever an effect causes a PC to risk corruption, they roll the corruption rating of the source of the effect. For each die higher than the PC’s current corruption they increase it by 1. Then, if rolling more than one die, add up all the dice rolled. If the sum is greater than their current corruption, increase it by 1 as well.

At the end of a full heal-up, current corruption becomes the new permanent corruption – heal it before that happens!

Corrupting Abilities

Following traits modify appropriate monsters or their abilities. They can be roughly broken into two categories: abilities that tempt PCs to risk corruption (always a choice), and abilities that hit you harder if you’re corrupted.

A Thing That Shouldn’t Be

(Apply to gibbering mouthers and the like – utterly aberrant)

To apply the escalation die to an attack against this creature, you must first risk corruption. Each. Time.

Insidious Violation

(Apply to attacks that inflict an effect with normal save, like mind flayer’s daze on mind blast)

Change the difficulty of the save to hard. Before making the save, a PC can choose to make the difficulty easy instead by risking corruption.

Maddening Visage

(Apply to boss-like monsters)

This creature gains a Fear Aura (no hp threshold), which can be ignored for a turn by risking corruption.

Unclean

(Apply to attacks that inflict a borderline unfair effect with a save. Give your boss a borderline unfair effect.)

Change the difficulty of the save to the target’s current corruption+.

Impossible Geometries

(Apply to attacks that are changed by the natural roll)

Change the natural roll trigger to “Natural roll equal or lower than the target’s corruption”.

Tainted Ground

(Environmental effect, think radioactive desert)

Spending an hour in this terrain causes a character to risk corruption. This check is repeated every 12 hours for as long as the character remains within the tainted ground. Corruption rating depends on the tier of the environment, and always uses 1 die.

Tear in Reality

(Environmental effect, an object on the battlefield: a ritual site, eldritch idol, etc)

Ending a turn nearby a tear in reality causes a character to risk corruption. Corruption rating depends on the tier of the source of corruption, and uses 1 die. Ending a turn engaged with the tear in reality (necessary to undo the ritual, study the eldritch idol, close the tear) increases the number of dice in the corruption rating to 2.

Effects of Corruption

Permanent corruption is broken up into several tiers:

0 – pure, good for you.

1-5 – mild corruption, cosmetic effects, can take corrupted feats.

6-11 – moderate corruption, this is really noticeable, but you get a free corrupted feat.

12-19 – severe corruption, really unpleasant effects. Have another corrupted feat as recompense.

20 – You’re an aberration now, time to make another character.

What are the actual effects of corruption? In a word, unsettling. Tentacles sprouting, eyes multiplying, transparent skin, shadow gaining a will of its own: all this and more. There are plenty of random mutation tables out there, and the boundary between gross and gross-yet-cool is very individual. You don’t want corruption to be so disgusting no one would ever wish to risk it. Damnation should be darkly alluring. All the mechanics have been written to tempt players into becoming corrupted, don’t let your description of its effects stop them. Work with the player to come up with a satisfactory description. It may be derived from the source of their corruption, or could manifest in entirely unexpected ways – corruption knows no rules (other than the ones written here).

Healing Corruption

Spend a recovery immediately after the scene where you became corrupted to reduce the current corruption by 1d4 points at the adventurer tier, 2d4 at the champion tier, and 3d4 at the epic tier, but never below the permanent corruption.

To reduce current corruption post factum, but before it becomes permanent, requires a costly ritual: material components cost 100gp if the target is Adventurer tier, 200gp if they are Champion, and 400gp if they are Epic. This ritual allows its target to heal current corruption as if they had just gained it. These components may not be readily available, however, especially in tainted ground.

Reducing permanent corruption is extremely hard, and likely requires a quest on its own.

Corrupted Feats

Regular feats, such as Reach Trick, can be reflavored to fit the corruption theme.

Whenever corrupted feats cause you to risk corruption, the corruption rating is determined by the tier of your corruption: d6 if it is mild, d12 if moderate, and d20 if severe; while the number of dice is equal to the number of times you’ve used this ability since the last full heal-up.

Forbidden Lore

You gain a new aberrant-related background, with 1 point in it if you have mild corruption, 2 if it is moderate and 4 if it is severe.

Unnatural Toughness

Risk corruption to gain temporary hit points equal to your current corruption.

Out of Sync with Reality

Once per round, risk corruption to roll a special save against any condition, even one you cannot normally save against. You succeed on this save if you roll less than or equal to your current corruption.

Vile Devastation

Risk corruption to increase the damage of your attack by your corruption. The target is probably corrupted as the result, too, not that you care. You monster.

New Regular Feats

These abilities could easily be granted by magic items, too. That’s probably a better idea if corruption is not the focus of your campaign.

Pure Soul

When you heal corruption, roll d6s instead of d4s.

Azure Flame Halo

When you risk corruption, add the escalation die to your current corruption.

Nature’s Rage

Once per battle when you hit a creature with a corruption rating, you may add the corruption rating to the damage you deal.

Corruption is a Choice

This is crucial, the main thing I’ve learned from using corruption in one way or another for years. That’s what these rules were written to reinforce. Often, it is a desperate choice between survival and damnation. But that’s what makes it meaningful, an effective horror element in an otherwise heroic game. It’s a permanent, or at least very long-term, consequence of player choice. Take away the choice, though, and you may ruin your players’ characters. Not everyone wants to have tentacles coming out of their character’s eye sockets. But sometimes you have to damn yourself to save the world.