The Harbinger should, obviously, only be played in mono-blue Merfolk decks, and preferably in control shells that focus on icing out the opponent from doing much of anything rather than overwhelming them with tons of creatures, just like Richard Garfield intended
(Scryfall note) Protection from a player means that you have protection from each object controlled by that player
So that $95 current selling price on average is definitely understandable, especially when you factor in the usual, high standard of card art
Although most valuable Charizard Pokmon cards sell for a lot of money, the Gym Challenge collectible has continued to fail to take off in price
Attunement fits this bill, but it combines with Replenish to bring all those discarded enchantments (you were discarding enchantments, right?) back into play in one fell swoop