Corruption is rare in the standard Strange Aeons adventure path, but there are two parts of the adventure path that can expose PCs to a corruption. As those instances are rare, I include only the basic corruption rules below. If it ever comes up and someone gets corrupted, I'll add additional info on specific corruptions and their game effects.
Official Corruption Rules
Even the most pure creature can succumb to tides of darkness. What begins as a minor malady or errant idea can grow into something malignant—a spreading corruption that can obscure your morals, cloud your judgment, and ultimately devour your soul. There are a number of different types of corruptions, from the hunger of vampirism to the horrifying transformation of the promethean. Living with a corruption is often a terrifying experience, but also offers the temptation of dark gifts. Hosts sometimes choose not to fight the corruption, but rather accept it and allow it to progress. These unfortunate folk either succumb to lust for the corruption’s power or attempt to control the stain of corruption and use its gifts for some greater good.
Each corruption features a general description, followed by its catalysts and manifestations. The catalyst section explains how a creature might contract the corruption, how it progresses, and how it can be cured. When a creature first contracts a corruption and its manifestation level increases, the corrupted creature gains manifestations. Manifestations carry both beneficial gifts and detrimental stains. Also, as the manifestation level increases, the stains and gifts of previous manifestations might grow more powerful. Lastly, as a creature’s manifestation level increases, so does the possibility of the corruption taking hold entirely, defiling its host forever.
A short list of terms related to corruptions follows.
Catalyst: The inciting incident that corrupted you in the game’s story, combined with ways you can progress toward total corruption.
Corruption: A dark manifestation of evil or alien influence that changes you over time.
Corruption Stage: A measure of how close you are to falling to your corruption. Think of your corruption stage as how much your soul has been altered by your corruption. At corruption stage 3, you succumb and are no longer a PC.
Gift: A benefit granted by a manifestation of your corruption.
Manifestation: A manifestation is a way in which your corruption becomes more prominent. You gain manifestations roughly every 2 character levels you live with your corruption. Each manifestation has both a gift and a stain, although you might not get both.
Manifestation Level: This is a measure of how much your corruption has changed your body and mind. Your manifestation level equals the number of manifestations you have.
Stain: A negative effect granted by a manifestation of your corruption.
Contracting a Corruption
The catalyst section of each corruption offers a number of possible methods for contracting that corruption. Once you contract a corruption, you immediately gain a manifestation.
Your manifestation level becomes 1, and your number of manifestations and your manifestation level can increase as described in Manifestation Level. You also draw closer to losing your soul, as represented by your corruption stage. This is described under The Corruption’s Progress.
You gain a manifestation of your curse when you first contract a corruption, and gain more at later levels. Each manifestation includes both a gift and a stain (though campaign variants can change how you acquire these gifts and stains as described in the Variants section below).
Many manifestations have prerequisites that limit them to characters farther along in their corruptions. A prerequisite marked with an asterisk (*) is another manifestation in the same section.
Unless stated otherwise, the DC for gifts that allow a saving throw is equal to 10 + 1/2 your level + your manifestation level (see below). You can have a maximum of nine manifestations.
Each creature with a corruption has a manifestation level, which is normally equal to the number of manifestations the corrupted creature has (see Variants for exceptions).
Sometimes gifts and stains become more extreme as a creature’s manifestation level increases.
You gain a new manifestation roughly every 2 levels. GMs can introduce corruptions as early as your 1st (character level. A standard rate of corruption starts with a PC gaining the first stain at 1st level, the second at 3rd level, and another at 5th, 7th, 9th, and so on. Because you are limited to nine manifestations, your manifestation level can’t exceed 9th.
This standard rate of acquisition is a guideline rather than a strict rule. Many corruptions feature story considerations that could speed up or slow down the process, and individual GMs can alter the speed to serve the campaign’s narrative.
When introducing a corruption at higher levels, a GM could accelerate the rate at which the first manifestations are acquired or grant multiple manifestations at once. In any case, the GM decides when a corruption progresses, not the PC.
The Corruption’s Progress
Every corruption has an associated saving throw. Each time you fail it, your corruption progresses to the next corruption stage. Each stage causes a more significant change within you, until you become completely irredeemable at corruption stage 3. When you gain a corruption, you begin at what is effectively stage 0, with no direct penalties. You must attempt a saving throw when you are being pulled toward darkness, and these saves are usually spread over a long period of time (often weeks or months). As such, abilities that allow or force rerolls (or rolling twice and taking the higher or lower result) can never be used on these saving throws, and temporary bonuses don’t apply on the progression saving throw, even if they are long-lasting.