The Secret Potion is a very important item in Pokémon Brilliant Diamond and Shining Pearl. If you want to make your way down a particular Route in the game, you will need to be able to cure a flock of Psyducks that need your help.
To get the Secret Potion, you will need to defeat the first four gyms, then speak with the Galactic Grunt in front of the great marsh at Valor Lakefront. You can find Valor Lakefront on Route 213. The Galactic Grunt will try to run from you, so follow him.
The Galactic Grunt will end up leading you all the way to the Valor Lakefront where you will meet Cynthia. Speaking with Cynthia will get you the Secret Potion. This isn’t the only time Cynthia will help you, as she will also give the Cut TM when you meet her in the north of Eterna City.
She might be helpful from time to time, but she is someone you will need to go toe-to-toe with later in the game, so don’t get fooled by her manners. Cynthia will also throw all manner of powerful Pokemon at you, so make sure you are leveling up your critters and building a solid party to take her on.