Chekov's Spell -- This spell has a two-fold effect. The first is to make the target item almost-unnoticeable to anyone not specifically looking for it. The second is that someone who has seen the item will remember it when they encounter a situation that would be perfectly solved by use of said item.
A dark necromancer has discovered a way of making the magical energy cost of resurrection less than the energy he can gain by draining their life force. Given a sufficient supply of mind-drained minions, ultimate power beckons.