Seize the Soul

The color restriction is the whole shape of this design: it kills anything that isn't white or black, which is to say it answers the threats the deck running it is least likely to field itself. That intramural courtesy is deliberate, and haunt is what makes the trade worth four mana. Removal with haunt asks you to pay once and collect twice: the spell destroys a creature and leaves a 1/1 flying Spirit behind, then exiles itself onto a creature you choose, lying in wait until that host falls and fires the identical effect a second time. The second destruction and second Spirit are the engine, but the trigger is involuntary; once the haunted creature perishes, the ability goes on the stack whether the timing suits you or not. That is the friction. You front-load a guaranteed answer and a body, then wager on a future death you only partly control. The optimal host is therefore a creature whose death is likely or coercible: something already committed to combat, something the opponent can't easily protect, or one of your own creatures you'd happily trade. Haunt a creature the opponent values and they will simply keep it home and starve the trigger. Read that way, the card is guaranteed removal-plus-flier now and a second removal-plus-flier later, priced entirely on how reliably you can engineer the death you're waiting on.