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 drop from Boss Pokémon or PokéLoot Chests.
A King's Rock can also evolve Poliwhirl into Politoed if Poliwhirl holds it while being traded. The King's Rock will be consumed in the process.
Unimplemented
As Slowking is not in the mod yet, nothing will happen when Slowpoke is traded while holding a King's Rock.