Wakeroot Elemental

The five-green activation cost is the whole tension here: at that price, the ability is not something you tap once for value and move on. It is a mana sink that turns a stalled midgame into a repeatable land-animation engine, converting every excess green source into a fresh 5/5 with haste. Because the effect untaps its target, a land tapped for mana can then be animated and swing that same turn, so a green deck flooded with lands gets to spend that flood on a widening board rather than watching cards rot in hand. Land animation has a long lineage in green: the trick has usually lived on the lands themselves (Treetop Village and its kin) or on enchantments that hit a single permanent. Putting it on a body, and letting one creature convert an arbitrary number of lands, changes the math: the Elemental is both a 5/5 beater and a factory that immortalizes your excess mana into threats the moment you can afford GGGGG. The animated lands keep their haste and their 5/5 bodies for as long as they stay in play, which means a single unanswered activation compounds across turns. The catch that keeps it fair is the ruinous cost per target, while killing the source does nothing to the lands already turned: this rewards the grindy, mana-dense green deck that has already won the resource war and simply needs somewhere to point the overflow.