The King's Rock is an important evolutionary item that players need in Pokemon GO.
The King's Rock allows trainers to evolve their Poliwhirl into a Politoed or their Slowpoke into a Slowking. The latter is the current objective due to the Pokemon GO event, "A Very Slow Discovery."
This event focuses on slower Pokemon such as Slaking, Snorlax, Slowpoke, and of course, Slowpoke's regional variants and evolutions. To complete the event's Collection Challenge, players will need a King's Rock to trigger one of those evolutions.
Pokemon GO: How to get a King's Rock for A Very Slow Discovery
Trainers can participate in A Very Slow Discovery from June 8th to June 13th in Pokemon GO. This is also the allotted time for trainers to complete the A Very Slow Discovery Collection Challenge.
The challenge requires players to catch a Slowpoke, a Galarian Slowpoke, evolve Slowpoke into Slowbro, and evolve Slowpoke into Slowking. Evolving Slowpoke into Slowking requires a bit more than just Candy.
50 Slowpoke Candy are required, but trainers also need to get their hands on a King's Rock. This evolutionary item is not exclusive to Pokemon GO or this event, but can be found easier through other event-exclusive tasks.
There are a couple of surefire ways for Pokemon GO trainers to obtain a King's Rock. The first is through a free event box found in the shop. This includes 20 Poke Balls, 10 Great Balls, and a King's Rock for no cost.
The item can also be acquired by completing a specific Field Research task. Event-exclusive tasks will pop up all-event-long after spinning a PokeStop. The particular one needed for a King's Rock is "Evolve a Pokemon."
It does not have to be a Pokemon that the event is centered around, either. It simply requires a Pokemon evolution to take place. Submit that Field Research task and a King's Rock will be given to the Pokemon GO trainer as a reward.