Following a discussion elsewhere on this forum, I have been motivated to put pen to paper and have a go at composing some magical corrupton rules.  I freely admit what follows is my own interpretation of S&S magic; the literature is doubtless full of exemplars of less drastic consequences for the magician.

Constructive comments welcome.

Magical Corruption for ASSH and other Retroclone RPGs
What follows takes inspiration - directly or indirectly - from Akrasia's Akratic Wizardry, Taint from the D&D Wiki, and Thulsa's The Spider God's Bride (my apologies for others I've forgotten).

Many of the effects of corruption start off as fairly cosmetic and function to add colour to the magic-using character.  However, as characters advance in power, effects of corruption will intrude more into the game. 

Magical corruption applies to all classes of magic using character.  Magic of any sort, be it the craft of wizards or miracles of priests, is inherently unnatural and draws its power from forces Not Meant To Be Known.  The toll on the sorcerer is both physical and psychological. 

To simulate magical corruption, the effect is based on the power of spells which the individual sorcerer is able to wield.  This reflects how deeply enmeshed the spell-caster has become in their eldritch dealings.  As such, those capable of only modest magics (such as Rangers) become only mildly affected whereas more powerful sorcerers can be heavily tainted.

In rule terms, the magic using character’s Corruption rating equals the highest level of spell they can cast.  So, a wizard capable of casting 5th level spells has a Corruption of 5, whereas one capable only of casting first level spells has a Corruption of 1.

A new Corruption point is gained when the character acquires the ability to cast spells of the next higher level (or when able to start casting first level spells for Rangers and similar).

When a point of Corruption is acquired, this manifests itself in some way.  Roll on the appropriate table below, re-rolling repeat entries. (Alternatively, the DM can choose or invent something suited to their game world):

Corruption of 1 or 2
1. Bad body odour (e.g. smell of the grave)
2. Sinister presence
3. Odd things happen in the vicinity (milk sours, animals stillbirth, eggs go rotten, etc.)
4. Cannot tolerate certain common materials or objects (rock salt, garlic, mirrors, etc.)
5. Unhealthy appearance (e.g. sallow jaundiced skin)
6. Odd behavioural quirk (e.g. nervous tick)

Corruption of 3 or 4
1. Natural (non-monstrous) animals dislike the magician (dogs snarl, horses smart, etc.)
2. Moderate bodily distortion (roll: 1. claw-like nails, 2. hunched back, 3. hair falls out in patches, 4. prematurely aged, 5. ears fall off or nose sloughs away, 6. weeping sores, 7. odd gait, 8. leprous skin)
3. Moderate psychological disturbance (roll: 1. develop a phobia, 2.hear disturbing voices talking to you, 3. see disturbing things intruding into the periphery).  These manifest at awkward moments; make a save against Transformation when under stress.

Corruption of 5 or 6
1. Major bodily distortion (roll: 1. a tentacle, facial parts, or similar grows from somewhere, 2. eyes become eerie points of light, 3. skin rots away, 4. cadaverous appearance, 5. monstrous or animalistic appearance)
2. Significant psychological disturbance (roll: 1. become paranoid (someone you know is out to get you), 2. intrusive obsession (you are being watched or hunted by something), 3. horrific visions of things intruding into the world.  These manifest at awkward moments; make a save against Transformation when under stress.

Sorcerers suffering significant manifestations can attempt to temporarily ameliorate or offset them by descending into depraved behaviour to appease the Dark Powers.  Remedies are inherently unsavoury, and include things like bathing or washing in blood, cannibalism, carnal depravity, and similar.  Generally speaking, the worse the behaviour or more obscure and outré the ingredient, the longer the salve will work (maybe a couple of months instead of a week or so).  Such a sorcerer will find themselves becoming outcast from society (or the locals will go after them with torches and pitchforks).  Thus, a powerful sorcerer may hide himself away in a wilderness stronghold, guarded by a band of hyena men, or in the obscurity of a big city with a loyal henchman to do his daily bidding.  To find less intrusive remedies, sorcerers may spend significant time and effort obsessively researching the power of strange ingredients such as the dust of long-dead kings.

Lesser (or less obvious) manifestations may be held in temporary abeyance through more mundane means.  Such remedies will be expensive or inconvenient, but not necessarily desperate, depraved or inaccessible by nature; for example, dusting one's skin with the powder of pearls, or having a tattoo ritually inscribed.

In extremis, a powerful sorcerer may attempt to defy their deterioration by possessing another person's body through vile and forbidden means (for example, some variant of Magic Jar).  However, this may buy only a few months or years; their tainted soul will eventually corrupt the new body.  Thus, such a fiend may spend their years 'moving' from body to body in an attempt to outwit the Dark Powers.  As a rule of thumb, time free of corruption is the age of the victim divided by corruption squared.  For example, a sorcerer with a corruption of 5 who possesses a person aged 25 will present normally for a year (25/(5 squared) = 1); the older the victim, the slower the corruption.

It goes without saying that engaging in depraved practices is Evil, and sorcerers of other alignments will shift to Evil.

Other effects of corruption
Those with a Corruption of 2 or more can declare themselves.  Their reputation precedes them or their presence betrays their nature.  When they declare their name or similar, anyone who wishes to take offensive action must make a saving throw (against Sorcery).  Failure means they cannot attack due to fear, superstition, or similar.  However, the effect is broken should the sorcerer take offensive action themselves (including use of spells) against the affected target. This influence affects only intelligent sentient earthly beings.  (Credit for this goes to Thulsa's The Spider God's Bride).

The lifespan of magic using characters starts to extend unnaturally.  The rate of ageing in years is divided by Corruption squared.  Thus, for someone able to cast first level spells, the passage of one year affects them normally.  However, someone with a Corruption of three will age only one year for every nine that pass.  Someone with a Corruption of 5 will age one year for every 25 years.  As such, a powerful sorcerer can live for centuries.  The extended life of high level druids and similar should be included - say by a factor of two; they have the potential to live for a very long time indeed (e.g. a druid with a Corruption of 6 will age one year for every (36 * 2) years).

Notes
DMs are encouraged to use their imaginations here.  Manifestations should perhaps be reflective of the nature of the magician.  For example, a druid may smell of dung, grow a mossy beard or hair, or (for something more significant) develop cloven hoofs or horns, or woody skin.  A psychological manifestation in a witch might be something like an uncontrollable or obsessive lust; the vicinity of a pyromancer may be afflicted by minor inexplicable fires; disturbing mirages and similar distortions may be experienced by those living near to an illusionist.  Look to the class descriptions for inspiration.

The results of corruption may be a bit extreme for some interpretations of sword and sorcery.  One option would be to apply the full effects to only sub-classes of sorcerer, or those who are especially evil; for other magic-users, cap at low or intermediate effects.

Finally, for other retro-clone games such as OSRIC, where spell levels extend to between 7 and 9, the corruption boundaries in the above tables could be substituted with something like 1 to 3, 4 to 5, 6 or above.