The Destiny Knot is a tool held quite useful in the Pokémon series not so much for its usefulness in battle, but rather for its efficiency in breeding Pokémon with good IVs. If a Pokémon holds the Knot of Destiny while being paired with another, it is guaranteed that five of the two Pokémon's total 12 IVs (each Pokémon has six IVs) will pass to the child; if both Pokémon have good IVs, the child will have it too. It's a good item to have in Pokémon Shining Diamond and Shining Pearl, whether you're looking to play competitively or want to build a group of maximum IVs that can take on the games' various post-match challenges. Here's where to find the Knot of Destiny in Pokémon Brilliant Diamond and Brilliant Pearl.
Destiny Knot is located on Route 224, which you can reach via Victory Road. However, the path for the path will be blocked until you get the National Pokédex. Once you have received the updated Pokédex, travel the previously blocked path.
When you arrive at Route 224, go up until you reach the beach, then keep right. Once you reach the end of the beach, there should be two trainers, each near a tree, north of you. Depending on how you feel, fight / avoid them and then climb a ladder to the left. There will be a Pokéball in the upper right corner of this platform which contains the Destiny Knot. Once acquired, you can give it to any Pokémon you wish to breed by equipping it as you would any other held item.