Cosmium Confluence
The charm-and-confluence template has a long history in Magic: three modes, chosen freely, each one a whole card's worth of effect at a discount. What makes this one specific is the terrain it's built to farm. The Cave typal theme is a strange thing to reward, because Caves are lands first: they do nothing on their own except sit in play and feed cards that count them. This confluence solves the archetype's chicken-and-egg problem in one cast. Search-a-Cave onto the battlefield tapped builds the board it needs; the +1/+1 mode animates one into a hasty attacker (a 0/0 base that only survives because the same mode staples three counters onto it, so the two clauses are inseparable); the enchantment-destruction mode is the flex slot for when the terrain plan doesn't need every point of gas. The freedom to repeat a mode is where the design gets interesting: three fetches assembles a Cave-heavy board in a single turn, three animations can turn a wide Cave count into a lethal swing, and mixing them lets you tune between developing and threatening. The whole thing is sorcery-speed and deliberately clunky at five mana, which is the price for handing a niche land-type deck its payoff, its ramp, and its interaction all in one card. It's a keystone by construction: the piece that makes a pile of otherwise inert lands into an engine.
