What is the goal here? To make spell casters even less useful in combat than they already are? If so, you'll definitely achieve that, with mages doing even less in battles than they already do, as they will be stood there, round after round, not contributing anything to the fight at all.I tried to articulate my goal Sunday, but I had just woke up and maybe didn't do a very good job of it. Here's the basic deal:
It might be simpler to tell your players that you don't like magic in your campaign - so don't use it!
1) Under most incarnations of the rules spellcasters risk losing their spells if they begin casting and are damaged before they are done.
2) My house rules on initiative provide for instantaneous casting, eliminating that risk that MUs must normally account for.
3) Therefore I'm looking for new ways to re-introduce the risk without scrapping my initiative rules.
My proposed chance of fumbling gives MUs a sliding scale of risk that the player controls. I like that. And I suspect that in a properly frantic campaign a spell going awry 1 time in 20 is probably more generous than the usual scenario of the goblins pincushioning the wizard with arrows before he gets his sleep spell off.
Also, I think it might be premature to judge my plan without seeing the fumble chart. Maybe I'll write a draft later in the week.
I think K. Bailey was on the right track in proposing that "instead of tracking the increasing negative modifier, may be easier to say that if you wait before casting, you halve the fumble chance (round down) each round waited." So if you want to throw a fireball you can choose the following options:
Instantaneous casting: 3 in 20 fumble chanceThe temptation inherent in faster casting greatly appeals to me. But again, whether a 15% fumble chance is fair or not really depends on how deadly the fumble chart turns out to be. We'll see.
1 round casting time: 1 in 20 fumble chance
2 rounds casting time: no fumble chance
Finally, the suggestion that I am anti-MU really made me smirk a bit. My needs-to-be-updated house rules document allows magic-users way more starting spells than baseline Labyrinth Lord, limited reuse of spells without re-memorization, easy-sleazy scroll creation from first level, and the option of learning to make potions starting at level 2. And just today I got done with the third draft of a one page list of leads for sources of new spells in my setting. If anything, I'm far too in love with magic-users to the expense of other classes.