in MR4,you fight tangerine who has energy blast,and scarlet who has energy mines, near the end. in the cutscene after that, merlin finds the golden bullets and has a dream that the golden bullets turned into the evil bullets, which after, teleports merlin to a small map in the dream, where you find the golden bullets.

in MR5, you fight through the first bit of the map as normal(it will be quite easy since you have the golden bullets), picking up the pheonix ashes(someone elses suggestion)but stronger. then,quite early in the game,the black sorcerer turns the golden bullets into the evil bullets,which turn merlin evil. the evil bullets vanish in a puff of smoke, which if you click on it(it takes about 10 seconds to disppear)will show you a picture of prestotolin and dellin(see dellin the brown)which appeares in your inventory. later on, the picture becomes a one-use powerup to summon the golden bullets. then you fight all the aldevar except prestotolin. ulin is the boss.

then, in MR6, prestotolin tells dellin what happened,making them the main characters for the level(you press alt to switch).
then later on, they become seperated, and prestotolin becomes the main character. prestotolin then finds the evil bullets, which have no affect on him.when you find it, you will be presented with the same picture, with something different. click the different bit to reveal a scroll. you read it, and it has a riddle on it, as well as giving you energy drain. then at a city, the gaurds will ask you for a password, which is the answer to the riddle. then in the city, there are many villagers, who give you information. one villager sells small pieces of cloth. the first few are useless, but one, with the picture from earlier on it, is useful. you then meet dellin, and you put the cloth on a discolored stone, which will teleport you to somewhere else. there, you find merlin, still evil, drop the phoenix ashes. use that to help the wizards and fight merlin and the BS. then, there is a cutscene with melrin the yellow sorcerer,but well save that for the next one.