TEXT: OVERCASTING 
  Casters of Arcane and Divine magics may 
  attempt to Overcast extra spells beyond those 
  they are allotted each day.  However, each time 
  that a spell is overcast, the player must make 
  an Overcasting Success throw.  Roll a d20 and 
  consult the Overcasting Success table. Failure 
  results in a fumble. 
  Arcane casters who fumble roll on the ARCANE 
  MISHAPS table. Divine casters roll on the 
  DIVINE DISFAVOR table. 
  Subsequent attempts to Overcast require 
  successively higher throws to succeed. The 
  Overcast throw is reset after a full rest. 
  For example, a 1st-level Wizard can normally 
  cast one 1st-level spell per day. If she elects to 
  overcast a second 1st-level spell the same day 
  before resting, she fumbles the spell on a d20 
  roll of 1-5. A third spell overcast before resting 
  results in a fumble on a d20 roll of 1-10. 
  One cannot overcast spells of a level to which 
  one does not already have access. However, 
  Clerics, Bladedancers, Dwarven Craftpriests, 
  Nobiran Wonderworkers, and other classes 
  built upon a Divine Value of 1-3, may Overcast 
  spells at 1st-level, even though they do not 
  normally have access to spell-casting until 
  2nd-level. Doing so is considered the first 
  Overcast of the day and requires a success 
  throw of 6+. 
  All casters may choose to spend double the 
  time overcasting a spell in order to gain a 
  bonus of +3 to the Overcasting Success throw. 
  In most cases, this means taking two rounds to 
  overcast a spell. The player must indicate that 
  he is taking double the casting time when he 
  declares that he is casting the spell. 
  Note that if the character takes damage or fails 
  a saving throw at any time during the cast 
  before the spell goes off, the spell is lost. In this 
  case, no success throw is made, but it does 
  count as an attempt. 
  ARCANE MISHAPS 
  When a caster of Arcane magic fumbles due to 
  Overcasting, he must roll d% on the ARCANE 
  MISHAPS table. 
  A mishap represents a mistake of some kind. 
  Perhaps the caster mispronounced a magical 
  syllable, or waggled his fingers the wrong way. 
  The caster may also have let his guard down, 
  allowing the extra-planar creature he had 
  bound to his will to gain the upper hand. 
  If the mishap results in a Corruption, then roll 
  d% on the CORRUPTION table. 
  DIVINE DISFAVOR 
  When a caster of Divine magic fumbles due to 
  Overcasting, she must roll d% on the DIVINE 
  DISFAVOR table. 
  Disfavor represents a transgression perceived 
  by the divine powers that the character serves. 
  It may be that she misspoke a prayer or 
  performed a holy ritual incorrectly. The caster’s 
  deity, or their intercessor, might decide to 
  teach the m a lesson. It may just be a rough 
  day in the Outer Planes. The gods are fickle, 
  callous, and pernicious, and they and their 
  representatives care little for the travails of 
  mortals. 
  Overcasting Success
  Attempt of the 
  Day
  Throw
  Throw w/
  Double Time
  1st
  6+
  3+
  2nd
  11+
  8+
  3rd
  16+
  13+
  4th+
  20+
  17+
  
  ARCANE MISHAPS TABLE
  All Arcane Mishaps of 75+ result in the spell cast being lost. 
  MISHAP [d%]
  RESULT
  01-50
  The spell takes twice as long to cast. If you stop casting, the spell is lost.
  51-70
  The caster simply fails to cast the spell. The caster may take no other actions this round unless she 
  has the Unflappable Casting proficiency.
  71
  Cast a different random spell of the same level from your repertoire.
  72
  The power of the spell is internalized. The caster immediately takes 1d4 damage. Until the caster 
  makes a Save vs. Spells at the beginning of his initiative, he takes an additional 1d4 damage.
  73
  Target self. If this wouldn't otherwise result in a negative outcome, the spell instead does 1d6 damage/
  spell level to the caster.
  74
  As target self above, but instead target a random ally within range. If this wouldn't otherwise result in a 
  negative outcome, the ally takes 1d6 damage/spell level. If there are no allies within range, then the 
  spell targets the caster.
  75
  Magical energy PBAoE blast of 1d6 damage/level of the spell. Radius is 5' per level of the spell. Save 
  vs. Blast & Breath for 1/2 damage.
  76
  A Dispel Magic spell is cast in a 5'/spell level radius centered on the caster.
  77
  The caster temporarily loses the ability to cast spells for 1d4 rounds.
  78
  The caster is knocked unconscious for 1d4 rounds.
  79
  Temporary loss of spell from Repertoire - regained the next day.
  80
  Temporary loss of a casting slot of the same level of spell that was being Overcast.
  81
  Lose casting ability of all spells until a full, uninterrupted 8-hour rest has been taken.
  82
  The weather within 1 mile per level of the spell radically changes for 1d4 (1-3) hours (4-5) days (6) 
  weeks per level of the spell. This can occur even if indoors or underground. See Control Weather for 
  some possible side effects.
  83
  The caster suffers memory loss of the last 1d3 (1-2) hours (3-4) days (5) weeks (6) years.
  84
  The caster is aged 1d4 years per level of the spell.
  85
  The caster is more prone to Spell Corruption, permanently increasing his Overcasting spell fumble 
  range by 1.
  86
  Permanent loss of the spell from the caster’s Repertoire.
  87
  Permanent loss of a casting slot of that level.
  88
  A wasting disease causes the caster to suffer -2 on all attack and Charisma rolls, naturally heals at 1/2 
  rate, and will die in 2d12 days unless he receives a Cure Disease.
  89
  Permanent Dispel Magic zone created centered where the caster is currently standing, 5'/spell level 
  radius.
  90
  Caster turned to stone as per the spell, Flesh to Stone. No saving throw allowed.
  91
  An extra-planar creature associated with the spell places a Geas upon the caster, no save. Until the 
  Geas is completed, the caster can no longer cast this spell.
  92
  A (random color) Sphere of Annihilation is summoned into the world. Roll a Save vs. Paralysis to avoid 
  destruction if it comes within 10'. Any attempt to control it for the first 1d100 days will automatically fail. 
  The caster will never be able to control it. The sphere moves 10' in a random direction (d10 - 9 or 10 
  same direction as last turn) and up/same/lower elevation (d6) each turn.
  93
  The life essence is sucked out of everything within a 5' radius per spell level. All living creatures must 
  Save vs. Death or be aged 1d4 years per level of the spell. All flora in the radius withers and dies. All 
  food within the radius spoils and water evaporates.
  94
  The spell pulls a monster into the world. The being will have HD, damage, and saves equivalent to a 
  monster the level of the caster +2. Roll 1d4 special abilities from the Undead Special table. The 
  creature will immediately attack, concentrating on the caster, if possible. Also, Lamentations of the 
  Flame Princess rulebook, pages 142-152.
  95
  The caster immediately explodes/implodes in a disgusting show of blood and gore.
  96-98
  The caster's body is corrupted. See the CORRUPTION table, below.
  99-100
  DM rolls/takes something from this table and applies it to everyone and everything on a large scale. 
  For example, the entire village suddenly succumbs to a wasting disease or all casters within a 1 mile 
  radius permanently lose a casting slot of the appropriate level.
  
  CORRUPTION TABLE
  CORRUPTION [d%]
  RESULT
  01-04
  The character must sustain themselves on something other than normal edible food. This could 
  be blood, metal, dirt, etc.... The character's teeth and digestive tract will mutate to accommodate 
  the new eating habits. Normal food now acts as a poison that does 1d6 damage and causes 
  sickness for 1d10 days.
  05-08
  An increase in metabolism results in all of your hair and nails growing very rapidly. Spend 1 hour 
  per day grooming or start looking like a freak. After 2-4 days, you suffer -2 to hit from your nails 
  and hair getting in the way. You also have to eat twice as much.
  09-12
  Roll for each hand and foot. On a 1-3 (d6) you grow an extra digit on that appendage.
  13-16
  You grow a 1-4' long tail. 1-2 Furry 3-4 Reptilian 5-6 Rodent-like. It is not prehensile.
  17-20
  Festering sores spread over your body. They are slightly painful, but otherwise cause you no 
  other discomfort.
  21-24
  Your 1-3 Upper Canines or 4-6 Lower Canines grow into 1-3" fangs. You may make Bite attacks 
  for 1-3 damage.
  25-28
  Your skin changes to scales. Choose a color!
  29-32
  You grow fins on 1-3 your back, 4 the backs of your arms, 5 the backs of your calves, or 6 the top 
  and back of your head. You may not wear armor in those locations (AC -1) unless you get it 
  custom-made.
  33-36
  Your tongue forks. Start adding hisses to your speech and slurring your S's.
  37-40
  Weakened musculature: -2 Strength.
  41-44
  Weakened immune system: -2 saves versus poisons and diseases.
  45-48
  Weakened physicality: -2 Constitution.
  49-52
  Swollen and stiffened joints: -2 Dexterity
  52-56
  You grow 1-3" Horns on your 1-4 Head, 5 Shoulders, 6 Elbows. These may be curled, straight, or 
  antler-like. You may use them to attack for 1-3 damage.
  57-60
  One of your limbs withers. 1-3 Arm, 4-6 Leg. A withered arm means you cannot climb, dual wield, 
  or use two-handed weapons. A withered leg requires the use of a crutch or peg, movement is 
  reduced by 30', DEX reduced or made worse by 1/3 for AC purposes.
  61-64
  Aged-looking. Your hair turns white (for men this means some baldness and liver spots) and your 
  skin sags. This is only for the sake of appearances -- your character has not actually aged.
  65-68
  Furry. You grow fur all over your skin.
  69-72
  You are now anemic and suffer an extra hit point each time you take damage.
  73-76
  Light Sensitivity results in you suffering a -2 to all attempted actions while in sunlight or bright 
  light conditions. Additionally, your skin changes to an alabaster color.
  77-80
  Odd skin color -- Purple, Jet Black, Alabaster White, Blood Red, Green, etc...
  81-84
  Unusual eyes - 1- Cats, 2- Amphibian, 3 - Insectoid, 4 - No irises (pure white), 5 - All irises (pure 
  black), 6 - Double irises.
  85-88
  Voice changes 1-3 Sound like opposite sex, 4-5 Pre-pubescent, 6 Otherworldly (double-voice, 
  hollow, booming, etc...)
  89-92
  Sex change - 1-3 Opposite sex, 4-5 Hermaphroditic, 6 Asexual.
  93-96
  You've grown vestigial wings -- 1-3 Bird-like, 4-6 Bat-like
  97-100
  DM's choice or imagination**
  
  DIVINE DISFAVOR TABLE
  DIVINE DISFAVOR [d%] RESULT
  01-25
  Your prayers aren’t worthy of attention. Perhaps if you continue for another round, you will be 
  heard. If you stop casting, your spell will be lost.
  26-40
  Your god’s agent has heard your plea, but is not satisfied. To cast the spell, take 1 hp/spell lvl.
  41-44
  The spell is lost. Perhaps if you pray more fervently next time the spell will take hold.
  45-48
  Someone's not happy with you. Prostrate yourself and fervently beg for a miracle or lose the 
  spell. You lose any positive DEX mod to AC and must spend a movement to later stand.
  49-52
  Rebuked! Lose the spell and suffer a temporary loss of ability to cast spells for 1 rnd/spell lvl.
  53-56
  Your prayers are rebuffed and you lose the spell. You may not attempt to cast this spell again 
  until you have rested  a full 8-hours and prayed as normal.
  57-60
  Doubts have arisen. Your spell succeeded, but you lose the use of one of your Divine class 
  abilities for 24 hours.
  61-64
  Castigation. The spell succeeds, but suffer wounds equivalent to 1d4 hp/spell lvl.
  65-68
  You have offended your god. Sacrifice treasure equal to 1/2 the amount of experience it takes 
  to your next level in gold within 24 hours. If you do not, you will be subject to Divine Disfavor 
  on a roll of 1 when spell-casting. Also, all Overcasting ranges are increased by 1. If you have 
  previously received this result, your Overcasting fumble range is increased by 1. Only the full 
  amount to the next level in gold value sacrificed will remove the penalty if it is incurred.
  69-72
  You are to be tested by the Downward Spiral. For the next 24 hours, any failed roll results in a 
  cumulative -1 to the next roll of that type.
  73-75
  Prove your faith. The spell succeeds, but for the next 24 hours, you may not wear armor, bear 
  a shield, wield a weapon, or use magic items. You may only call upon Divine spells in battle.
  76-78
  Enemy on the Other Side. The spell succeeds, but the next roll you make on the "Tampering 
  with Mortality" table is at a -1/spell lvl for the d20, and a -1 on the d6 for the Side Effect roll.
  79-81
  Vow of Silence. The spell is cast, but it is the last thing that you will utter for 24 hours. No 
  talking, turning/controlling, or spell-casting. Breaking the oath will result in the DM being able 
  make any one roll you make in the future any result that he desires.
  82-84
  Forsaken! The spell is lost. The misapplication of your god’s power results in a Forsaken 
  Sinkhole of Evil being formed in a 50'/spell lvl radius. See ACKS pp 251-252. The misdirected 
  energy of the spell immediately causes 1d4 Skeletons/spell lvl to rise from the ground. These 
  undead will be spread out evenly and attack the non-undead creatures closest to them.
  85-87
  Mission from God. The spell succeeds, but the caster is placed under the effects of a Quest by 
  an agent of their god equivalent to lvl 8+1d6. No initial save. The nature of the quest is for the 
  betterment of the faith, and often involves the return or destruction of some relic.
  88-90
  The Mark of Shame is apparent upon the caster. The spell fails. All those who venerate the 
  caster's god will have no dealings with the caster. Clerics, Priests, Paladins, and the fervent of 
  faith may seek to do you harm; physical or otherwise. Enemies of the faith will only take 1/2 
  dmg and receive a +2 to their saving throws from your spells. The Mark will only be removed 
  when the caster has sacrificed something (or someone) of great personal value.
  91-93
  Represent! The spell succeeds. You now have one week to convert one new member to the 
  faith/spell lvl. Only NPC conversions count. Use the "Reaction to Hiring Offer" rules on ACKS 
  pg 49, with a result of "Accept" or better resulting in a success. Failure by the end of the week 
  means that you suffer a temporary loss of one spell-casting and turning level per recruit that 
  you came up short until you succeed.
  94-96
  Something slipped through. While the spell succeeds, the power used to perform the miracle 
  loosed an entity from its shackles and it has escaped into the world. It is now possessing a 
  nearby creature and can pass from one creature to another through touch. It does not have to 
  inhabit the body of a living creature and can exist without a host, though it has no powers of its 
  own. It will seek to destroy the caster at the most inopportune moment, likely striking through 
  someone close to them. A single Save vs. Death is allowed for anyone possessed in order to 
  throw the entity out. It may not inhabit that person again if the save is successful.
  97-99
  Blue Bolt! You are smote on the spot, dropping immediately to 0 hit points.
  100
  The shadowy servants of Avod, Goddess of Death, erupt from the ground and pull the caster 
  into the Underworld. The caster is trapped there unless someone can devise a means to 
  rescue him.
  
SOURCE: textdata/thevault/Adventurer Conqueror King/ACKS Patreon Content/Overcasting Arcane Mishaps, Corruption, & Divine Disfavor v1.1 (ACKS).pdf