The evoker is a self-protected item found in brain biomes that can't be destroyed by normal means of destroying. If the evoker is still active, it will cause brains to spawn at random intervals at players. A single player alone on a brain world will be swarmed by 5 brains every 10 minutes until all evokers are destroyed.
How to Destroy
The evoker is destroyed by placing an inhibitor nearby and connecting it to a steamline with brass pipes and a collector. The anchor block of the inhibitor must be within 5 blocks of the anchor block of the evoker to be destroyed. Be careful, however, as a chain of explosions will happen and destroy both the evoker and inhibitor.
The evoker was added as part of the brain biome update.