Psaro after having used the Secret of Evolution to change forms.
The Secret of Evolution is a key plot element in Dragon Quest IV. It is said to be integral to the development of all life everywhere, but extremely dangerous if it falls into the wrong hands.
History
It was originally discovered by Esturk, King of Hell, a thousand years before the game begins. Esturk uses the Secret of Evolution to transform his body and become the ultimate life-form, and in so doing becomes the most powerful being on the planet. However, although he is massively powerful as an individual, he is not insurmountable, and a group of humans are somehow able to overcome him and trap him in a tomb deep beneath Aktemto.
After this, the secret was lost, since Esturk had wisely kept it to himself, but was rediscovered by Mara and Nara's father, the alchemist Edgar. However, his apprentice Balzack murdered him and stole his research, using the imperfect version to empower himself.
Balzack then passed on what he had learned to his master, Saro, who held off on making use of it for fear of losing his sanity; however, when Radimvice has his lover Rosalee murdered, he flies into a rage and performs the Secret, taking on a form very similar to that of the transformed Esturk.
When the Hero and his party confront and kill Saro, all knowledge of the Secret of Evolution presumably dies with him.
Usage and Effects
The exact steps one must take to perform the Secret are unknown, but it can be assumed that it makes use of magic or is even a form thereof. The technique acts to physically alter a person's physical characteristics, generally making them larger and stronger, and often adding other bonuses as well; for instance, Esturk and Saro both acquire an armoured carapace.
For some reason, the Secret of Evolution requires the Golden Bracelet in order to work as intended; if attempted without it, the results are imperfect and often unpredictable. Keeleon, for instance, became a more lion-like monster, and Balzack became huge and bloated. Each of these forms has a weapon unique to it - Esturk and Saro use two swords, Keeleon uses bared claws, and Balzack uses a club - but whether this is an effect of the transformation or simply a matter of personal preference and common sense is unknown.
In the remake, Rosalee's tears were able to reverse the effects of the Secret of Evolution on Saro, perhaps due to her heritage, her own unique magical traits, or her love for him. There may be other circumstances necessary in order to make this reversal possible as well.