I've been researching some alternative magic systems for my D&D campaign, which I'd like to have a lower-than-normal level of magic, with an element of risk. The system in the Thieve's World PHB looks really good, but I do have a question / concern and I'd like to know what other people think.
When casting a spell, the unmodified die roll is checked against the mana level of the area. If the die roll is low enough, then mana is subtracted from the caster's mana pool. If the pool ends up negative, then there is a spectacular failure with the potential of some pretty severe consequences.
If I'm reading the rules / tables correctly, it seems that there is a spectacular failure 25% of the time in normal areas, and 50% of the time for low-magic areas, whenever a caster starts to cast a new spell. Not just failure - but earthquakes, curses, and severe penalties. Yikes!
Was the casting system intended to be this risky, am I reading the rules incorrectly, or are the rules for preparing spells in advance supposed to be the standard way of spell casting (with casting-on-the-fly a last resort)? Does anyone have suggestions for making the system a little less volatile?
I'm really enjoying the Thieve's World take on things in general and I'm considering adapting a lot of the material in there for my game. In many ways it's the D&D I've always wanted.