Where an individual spell has its own failure table always use that one first, coming back to this one if referred. Otherwise just use this one straight. I often use the shorthand of "smoke, nightmares, and witches" to explain the spheres Moon Slave governs. This does not mean it is a god of all magic or that he is some source of all magic in a Dragon article kind of way. It does mean that he wants more magic and more powerful magic in the world. He takes magic's use, especially dangerous magic or magic ill-applied, as a form of worship. It will not be denied the spread of his worship just because YOU fucked something up. There is no "the spell just doesn't go"/"you lose the spell" option.
My spell failure is going to have three extra rules:
1) If you survive until your sixth spell failure an aspect of Moon Slave shall appear in a plume of flame. It will cast one spell of its own and then vanish. You're on warning.
2) Performing a blood and burn sacrifice to Moon Slave beforehand lets you modify your roll against spell failure. This bonus equals the number of living beings with Hit Dice you just sacrificed. No partial hit dice like rats or worms.
3) If you kill a creature in 1 blow and by your own hand then you get another Safe Spell for the day.
On your 66th spell failure congratulations: Moon Slave appears and begins a slow march to claim you.
1. Something in the environment which would not normally burn catches fire for 1d6 rounds.
2. Caster loses 10kg. This is not so bad at first but repeatedly getting this result will have you slowly waste away from magic. No amount of pastry will fatten you up when magic eats you from the inside.
3. Area fills with choking smoke, Poison save to keep from being helpless while hacking and retching, completely obscures vision, lasts for a minute or until otherwise dissipated.
4. Spell is cast but it dulls all blades in your party, -1 to hit and damage until each blade is sharpened again.
5. Caster falls asleep for 5 minutes or until awakened, has horrible dreams.
6. Any coins you have melt, any paper money or marques you have burn.
7. Caster's name changes, but they don't know what their new name is. Their party members have always known them by this new name. Cannot cast spells if you don't know your own name.
8. -1 Constitution until your next bad dream. This stacks over time.
9. Full darkness. If torches or lanterns are lit they snuff, if the sun is shining bright above the sky instead appears like starless black. 1 mile radius.
10. Spell succeeds but treat as immediately triggering a wandering/patrol failure. You've got company.
11. Bat swarm, obeying no master.
12. The opposite of this spell's effect occurs. Light becomes dark, fire becomes ice, death becomes life, lightness becomes crushing gravity, etc.
13. Spell targets a random creature in line of sight, even if spell can normally only affect the Caster or a touch target.
14. A weeping apparition appears and follows you, soundlessly screaming. Every time you get a spell failure between now and the next black moon another will appear whether you roll this result or not. Everyone can see them but you.
15. A dead person returns to life - mostly - for a day. They know and will tell that you are the cause.
16. 13 witchfinders appear, confused and angry.
17. Hurricane storms manifest, out of all control. If this happens in an enclosed space you're pretty screwed.
18. Spell debt: lose one Safe Spell from your next alottment. Spell still fails.
19. Spell casts. Roll for a random spell. That spell also casts, affecting the same target or affecting a random target
20. Spell fails. Summon, Wish, and Permanency cast instead, in that order.