An example of using events to implement an achievment system

Achievements.cs

public class Achievements : MonoBehaviour
{
   //..More code here
   private void Awake()
   {
       Item.OnCollectedItem += HandleItemCollected;
   }

   private void OnDestroy()
   {
       Item.OnCollectedItem -= HandleItemCollected;
   }

   private void HandleItemCollected(Item item)
   {
       if (item is SpecialItem)
       {
           UnlockSpecialItemsAchievement();
       }
   }

   private void UnlockSpecialItemsAchievement()()
   {
       //..Unlock achievement
   }
}