So v18 and v19 both use a new battle system, which includes BattleHandlers for abilities (and items) that all trigger at the same time overall in the turn phase or have similar triggering circumstances, like all effects when dealing damage, or on losing enough HP. Stuff like that.
The easiest way to make new effects, like always, is looking at abilities (or even items sometimes) that do what you want already.
Crooked King is a blend of Poison Touch's UserAbilityOnHit and a PriorityChangeAbility (there aren't any that do exactly what you want though, since that trigger doesn't know who it's attacking, just the attacker and the move they were going to use.) We can cheat a little on the latter part, by sneaking through the choices on the battle object through the battler object.
battler.battle.choices[battler.index][3] is the target index selected, though In single battles, it's always -1, so the target depends on the move's target, and even in larger battle sizes, the target index might not be what you expect, so some experimentation would be needed.
Gallant Soldier would use an AbilityOnSwitchIn, and it'd need to loop over
battle.eachOtherSideBattler(battler.index), checking for the types of these opposing pokemon. While there isn't really an ability in this category that does something similar, Download, Forewarn, Anticipation, and Intimidate all use this method to loop over. You'd need a variable called like
found = false and then inside check if the type is one of them, set found to true and break out. Then if found is true, do the ability splash, and raise the two stats. This is probably the easiest to make of the three, and a good one to start with.
Violent Rage is a bit more complicated because it has effects that aren't covered by a BattleHandler. Almost all passive damage sources call
takesIndirectDamage? to see if they can actually deal damage (the remainder are the weathers, sandstorm and hail, which have their own methods, just below the indirect damage definition). If you don't care about passive damage specifically, and can do any type of damage, there's AbilityOnHPDroppedBelowHalf, but that's when HP drops below half. For the other half, ignoring the effects, the ignore burn atk drop is like Guts and you'll find that in Move_Usage_Calculations (you can change the
!user.hasActiveAbility?(:GUTS) to
!user.hasActiveAbility?([:GUTS,:VIOLENTRAGE]) to copy the guts effect for this one bit). The ignore paralysis speed drop is like Quick Feet, and you'll find that in
def pbSpeed in PokeBattle_Battler.
Since you want to learn Ruby, I do have to suggest some Ruby tutorials.
(broken link removed)
(broken link removed)
https://www.tutorialspoint.com/ruby/
Quick tip, CTRL + SHIFT + F lets you search all script sections at the same time, which is great for finding the definitions of effects, or just to see how a method is used or the variables it expects.
Best of luck, and feel free to ask for help!
