Hunter's Vengeance is an epic spell that deals damage to a random unit of each unit type. Upgrading it will increase the damage done and lower its mana cost. Hunter's Vengeance can see fit into most decks that revolve around a single unit type.
Hunter's Vengeance will deal more damage the more unit types there are on the board.
Be wary of Hunter's Vengeance's chance to damage friendly units when it is played. If you want to circumvent this, you can use a deck that contains very few different unit types.
Most notably, Hunter's Vengeance can counter decks that revolve around Ubass the Hunter. Their decks will contain lots of different unit types, which will only increase the amount of damage Hunter's Vengeance will do.