Do any of you guys have the strategy guide, seriously? Every time you use Valor, Wisdom or Master, one point is added to your unviewable "dark point total". Using final form lets you subtract 10 "Dark points", and using anti-form subtracts 4.
acquiring a form resets the counter to zero.
0-4 pts, 0% chance of change
5-9 pts, 10% chance
10+ pts, 25% chance
Certain battles, like scripted battles, change the odds (percentage-wise) by x2, and the final fight against
Xemnas multiplies it by 10.