The Staff of Mistery says "spells that create mists, fogs, vapours and miasmas are one category easier". Fog based battlemagics are explicitly easier than other battlemagics. It would be very strange if that only applied to casting the spell, and not also for creating the spell.
There are several questions here.
1.This update lays out explicitly that there are different tiers of batlemagic: Low/mid/high, edging into cataclysm. Presumably, based off tabletop, low battlemagic is easier to cast than high. Does (should) the staff of Mistery simply reduce all of these tiers of battle magic to fiendishly complex?
2.Assuming that the answer to 1 is yes, only FC. Spell creation is harder than spell learning. How much harder does that make the rolls. PUlling numbers from thin air, if the roll to learn a new FC spell was 80, an existing battlmagic was 100, what level of additional difficulty does creating the spell impose? +10 (still easier than learning existing battlemagic) +20 (same difficulty) +30 (actually harder, but reduced penalties for failure due to staff) etc?
2. Assuming that the answer to 1 is yes, only FC. Well sure - battlemagic miscasts are generally (or at least potentially) worse than regular miscasts. But how many times are you rolling the dice to get there? Learning an existing battlemagic spell requires making one difficult roll.
Creating a new spell might require multiple AP so say
3 or more moderately difficult rolls, each of which has the potential for a miscast and nasty effects.
Moreover, it is considerably easier to persuade the thread to shake loose a +20 from the gambler for one difficult roll than three moderately difficult rolls. So the odds change again.
Why do you assume this, when we've been explicitly told you can always miscast when learning an existing spell? Why exactly is making a new spell (which may be staff-compatible) be more dangerous than learning an existing one?
Because that is how it works. We have always been told that spell creation is considerably more risky than learning existing and already codified spells. Beyond that, creating a spell involves multiple actions and rolls to do so. Learning an existing spell involves making less rolls, so there is less chances for a low roll to occur.
Now there is a legitimate question about if the Staff of Mistery means that the added difficulty from blazing a new magical trail still adds up to less than the difficulty of learning an existing battle magic, but intentional spell creation has always been harder than learning existing spells.
