Whomever refers to anyone at all. It's often used when we don't know who the person is yet.
When we use "whomever" it usually means we are making a statement that will apply to any person, without caring about their name or identity. For example, if someone asks, "Whoever wins the game gets a prize," they're saying whomever (the winner) will receive the reward. In this case, "whomever" is used as an object of the verb and replaces whoever might have been used as subject.
Whatever person or persons (as object of verb or preposition).
Example: The letter is addressed to whomever it may concern.
