Thief is a dark-type move that can earn you some decent items from opponents in Pokémon Brilliant Diamond and Brilliant Pearl. However, you'll need to catch it in Eevopolis first to get this cheeky in-game attack.
Eevopolis can be found quite early in the game. It is located north of Oreburgh City and east of Eterna Forest. After obtaining the hidden ability Size, go to the Team Galactic Building which is located northwest of the city. Instead of going into the skill, turn right and you will see a hidden tree at the top right of the area. Cut it like the three trees before it. Follow the gravel path to find the TM46 Thief lying there for you to collect.
You only have one ThiefTM so be careful which Pokémon you give it to. We recommend giving this to a Dark type as moves of the same class will work more effectively than those of other types. In addition to stealing the item held by a target, it also has a power rate of 60 and an effective accuracy rate of 100. This would be a great secondary move if your primary attack like Bite runs out of PP.