Downhill Charge

Pay and the spell scales with every Mountain you control while leaving your board untouched; sacrifice a Mountain to cast it for free and the very act of paying shrinks X on the spell you just cast. That self-defeating loop is the design's whole engine. The free mode is rarely the bigger pump, because the land you pitch was a land that fed the count, so the sacrifice route only pays off when you stagger it: hold the sacrifice for a turn when the swing matters more than the land, or feed it a Mountain you were never going to spend anyway. The mana mode keeps your resources intact and lets the bonus grow with a fully committed red board; the sacrifice mode is the emergency lever, a way to push lethal through with no mana open and an empty hand. The friction is what stops an instant-speed pump of arbitrary size from being a clean kill on its own. Most combat tricks ask which creature; this one asks which resource you are willing to part with, and that answer changes the size of the effect before it ever resolves. It belongs to a small family of early-era spells that let you cash a Mountain to dodge the mana entirely, but here the gimmick eats itself: the more Mountains you pour in, the sharper both edges of the knot become.