campfire/Assets/Scripts/CampfireFog.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class CampfireFog : MonoBehaviour {

    public GameObject Player;
    public MeshRenderer FogMesh;

    void Update() {
        FogMesh.material.SetFloat("_Transparency", Mathf.Min(Mathf.Sqrt(Mathf.Max((transform.position - Player.transform.position).magnitude - 15, 0)) / 100, 0.15f));
    }
}