Well, in my honest opinion, a final boss should be a being that gives the player a sense of urgency and fear when they are there. He/She/It should just naturally give off an over powering feeling, but at the same time give the player a feeling of wonder and question. A final boss doesn't have to be the 'strongest' enemy you face, but they should use their talents to make it appear so. Either by using a specific strategy or skills set to pressure the player to focus not only on Attacking, but healing, buffing, debuffing, and other things of the like. It should have ways to overcome the player even at their full potential. Even if the Final Boss isn't the strongest enemy, the feeling of urgency and pressure should be enough for the player to understand it's position in the game without it having to be some un godly beast from hell, spawned by a crazed human/being. Hell, the final boss could be a twisted child and still place fear into the hearts of the challengers if done right. It's all in the eventing and how they are introduced, and flowed throughout the game. That's my idea of a Final Boss.