Leyline of Resonance

Every Leyline lives on the same wager: draw it in your opening hand and it costs nothing, arriving before the game has drawn breath; draw it any later and you pay full price to hard-cast it, at which point the enchantment is a slow luxury rather than a head start. The wrinkle here is what that free start buys. Where the older Leylines resolved into static hate pieces or passive anthems, this one sits inert until you cast a spell, then duplicates it. The copy clause is drawn tight on purpose: only instants and sorceries that target a single creature you control, which walls it off from burn and removal aimed at the opponent's board and points it squarely at pump and protection spells cast on your own threats. Because you may choose new targets for the copy, a single Giant Growth can grow two creatures instead of one, or a protection spell can shield a second attacker mid-combat. A no-mana, no-card spell doubler would be oppressive if you could reliably slam it on curve, so the design routes all of that power back through the opening-hand coin flip that every Leyline pays with. Miss it and the four-mana card rots in hand; open it and you begin with a copy engine already switched on, each relevant cast quietly resolving twice.