Jace's Mindseeker

Theft dressed up as mill: five cards off an opponent's library, and the right to fire any instant or sorcery you turn up for free. That ride-along clause is what pushes a 4/4 flier to six mana rather than four; it is upside priced into the body, and the trade Wizards struck is variance for power. You are not selecting from those five cards by any criterion except what happened to be on top of someone else's deck, so the spell you get might be a backbreaking removal piece, a sweeper, a counterspell with no legal target, or nothing castable at all. The mill keyword is doing color-pie work too: blue rarely gets to cast an opponent's spells, and tying the effect to their library rather than yours keeps the power contingent, since it scales with how spell-dense their deck is and craters against creature-heavy lists. There is a sharp edge to "without paying its mana cost," though: an X-spell hits the stack with X locked at zero, so the dream of a free Sphinx's Revelation resolves for nothing. The reward lives in fixed-cost spells, where a stolen wrath or counterspell or burn finisher arrives at no charge. The body is the floor that makes the gamble palatable: even on a whiff, you have an evasive four-power threat. The catch worth naming is that the trigger dumps five cards into an opponent's graveyard, so against any deck that wants its bin full, you are filling it for them.