A King's Rock is a held item that causes the holder's damaging attacks to have a 10% chance of causing the opponent to flinch. This effect does not stack with moves that already have a chance to flinch. It can be obtained as a tier 2 special drop. It is also a drop from certain wild Pokémon.
A King's Rock can also evolve Poliwhirl into Politoed and Slowpoke into Slowking if held while being traded. The King's Rock will be consumed in the process.