If you're talking about the first spell for Black Magic, it is this:
Offset: Fire Spell, Level 1 Magic
61A58 10 64 19 00 00 08 00 2F
Byte 0x0 - 10 is the element, in this case, Fire.
Byte 0x1 - 64 is the % of hit, 64 being 100% obviously.
Byte 0x2- 19 is the spell power, in decimal should be 25 if my brain works today. Offset is 61A5A.
Byte 0x3 - 00 Is the type of spell, in this case is "normal damage" type.
Byte 0x4 - 00 It seems to point to the target status, but I didn't play with it enough.
Byte 0x5 - 08 This is used to know what kind of targeting does, like, for example, 08 means is single target, but you can multi target it (By dividing the damage between the number of enemies), or 4F which is "Attack all with full damage" for example.
Byte 0x6 - 00 Data Crystal didn't know what it is, but it actually changes the animation for spells! If you change it to 02 it will simulate a Quake animation using the original spell's sprites , and while it look garbled it won't screw up the game. Kind of fun to play with. 03 is the Meteo animation, and so on. 00 uses normal animation.
Byte 0x7 - 2F is the class usability Index, which I didn't touch at all, each level of spells has it's own value.
About the stealing/drop thing, I though the same, but when I looked into it, well...http://shrines.rpgclassics.com/nes/ff3/enemiesp.shtml
Enemies seem to drop and carry different things, so no luck there. But now you gave me an idea: I'll try to see if those A6 values I see around 21B30 and 21B90 could be the Potion item, because there are too many A6 there, and there's too many potions as monster's steal items , maybe changing them for something easy to see like a piece of equipment will work in some way.-SUPER EDIT-
It ends up, I was right! I never thought it would actually work, what the heck...
Changing the byte in 21B91 (A6) to anything else (I used 37 -Masamune as an example) will give you that item while stealing Goblins! Although, you still steal potions, so they must have several slots with Potions to steal.
It's kind of weird, for example Goblins use 21B90, 21B91, and 21B92. The three have potions, and you'd think that you can change them into anything, but it seems that the game can ignore one of them if it doesn't find a Potion to steal depending on what item is in each slot.
If you put three different things in each slot, maybe the middle one is ignored in favor of Potion. But, if you put something in 21B90 and 91, you may steal them alongside the Potion on 92.
Which I guess it means that one, Maybe I modified Item Group 20, which only give Potions, or two, every Item Group reads several offsets and if I change one thing, every group that has that thing will change. I need to keep trying. Thank you so much.-EDIT 2-
You were right too, Drops and Steal items are the same, a Goblin just dropped a Masamune (How broken is that?) That's pretty interesting....
You can even get several items at once , I just got a Dream Harp and a Masamune in the same fight as drops, and the chance of dropping the items is okay. The thing is that very few enemies leave items when defeated, so it will be tricky to see which ones i give something, but that's another question entirely.
And thank you for taking your time for looking into it too. You don't have to, really, and if you don't see anything it's no problem, so don't feel obligated.
PS: The animation thing is something that surprised me. Even weapons have an animation bit and you can do crazy things with it, like changing axes' or spears' animation to boomerangs, and you see the actual sprite rolling around the screen doing circles.
The game seems very "resistant" to this things, even if you screw up the spells the game won't crash, it just do whatever the values say and continue, even if it's nothing at all or the effect doesn't exist (Like stealing, if you point to a value that it's not valid it will just say "Can't steal" and the game continues like nothing).