When Ferrafor enters, create a number of 1/1 green Saproling creature tokens equal to the number of counters among creatures target player controls.
: Double the number of each kind of counter on target creature.
Ferrafor is a mono-green counters engine: drop creatures loaded with +1/+1 counters (and keyword counters), then tap Ferrafor each turn to double every counter on a single target, exploding power, toughness, and abilities. The ETB also rewards a counter-heavy board by spawning a wave of Saprolings. You ramp into Ferrafor, build a counter base, and snowball one threat into a lethal beater while going wide.
Repeatable counter doubling turns modest creatures into one-shot kill threats
ETB Saproling generation gives instant board presence in a counters deck
Mono-green gives easy ramp, fight removal, and the best counter-synergy package
Plays well with proliferate, infect/toxic, and keyword counters for layered value
Cheap to assemble a focused, resilient gameplan around one engine card
Seven mana and summoning sick the turn it lands; doubling is slow to start
Tap ability targets only one creature per turn, so it can be answered with spot removal
Heavily reliant on having counters already in play to be impactful
Mono-green struggles to interact with combo and protect against board wipes
Doubling is useless if Ferrafor is removed before untapping