Mossborn Hydra

Doubling is the most explosive counter operator green owns, and welding it to landfall converts the most dependable trigger in the game into a growth curve. A creature that arrives with one counter goes to two on the next land, then four, then eight, then sixteen; two land drops and you are swinging for four with trample, and a fetchland or an extra-land-drop enabler stacks triggers within a single turn so the escalation stops being additive and becomes geometric. That is why the effect keys off lands entering rather than any permanent: land drops come often enough to snowball but stay scarce enough (usually one per turn) to keep the doubling from resolving all at once. Trample is the closing valve, not a bonus. A doubling clock that could be chump-blocked in perpetuity would be a threat that never connects, so the keyword pipes the accumulated counters straight into damage the moment a single blocker fails to absorb them all. What the design demands of a builder is not synergy in the abstract but density: extra land drops, ways to replay lands, and a plan for the turn the doubling outruns the board entirely. The counters are removable and the body begins at zero, which makes the bargain stark. Left alone, it ends games in three or four land drops; answered at any point along the curve, it collapses back to nothing.