Introduced in the second generation of Pokémon, Unown is a rather strange Pokémon. Despite being a single species, the Pokémon has nearly 30 variants, each of which resembles the letters of the modern Latin alphabet. Here's where you can find this strange and varied Pokémon in Pokémon Brilliant Diamond and Shining Pearl.
The Unown are, fittingly, hidden within one of the most mysterious areas of Brilliant Diamond and Shining Pearl: the ruins of Solaceon. You can reach the ruins from Solaceon Town by following a path in the trees behind the town's Pokémon Center. Continue right along the path to the end; jump the three small obstacles below you and the entrance to the ruins will be on your right.
Once inside, you will be faced with multiple stairs, some of which lead to dead ends, while others allow you to venture deeper into the ruins. The ruins are filled with Unown, and one should ambush within a minute or two of exploring. If you want every type of Unown, you'll be able to catch 26 variants of the Pokémon here via random encounters, but be aware that this will take a good amount of time and patience.
There are two types of Unown - the ones in the shape of an exclamation point and question mark - that you won't be able to get at first, but can be obtained after some footwork. After capturing the first 26 variations, visit the Ruin Maniac and you'll find that it has dug a tunnel from Route 214 to the Solaceon Ruins, allowing you to access a portion of the ruins that you previously couldn't. Here you can complete your Unown collection.