Leyline Immersion

The "enchant legendary creature" restriction is the whole design, and it names its own audience: a single named creature, recast game after game, is exactly the target that can absorb this kind of acceleration without the acceleration breaking anything. Five mana of any color every turn is explosive ramp; the card pays for it by tethering that output to a body the rules already limit to one copy. The mana carries a spend-only-on-spells clause, which walls it off from activated abilities and mana sinks and points every point of it at casting: a big turn, a chain of spells, a haymaker that outpaces what four mana up front should ever buy. Ward is worth reading precisely, because it does less than it first appears to. The Aura grants ward only once it resolves, so it does nothing to stop the classic two-for-one blowout: kill the enchanted creature in response to the Aura on the stack and both cards go to the graveyard, no tax paid. What ward protects is the second turn and every turn after, the point at which the investment has already survived and the opponent wants to answer a mana engine that has started paying off. That is the deal the card offers: no safety on the way down, a real premium on removing it once it is running. Everything else, the enchant clause, the spending restriction, follows from wanting all that ramp fixed to one creature.