Primordial Sage

The card-on-cast trigger is the engine, and the choice of "creature spell" rather than the broader "spell" tells you exactly what kind of deck this was built to anchor. It is a draw engine that rewards going wide and going deep on bodies: every dork, every token-maker, every fatty you resolve refunds itself with a card, so a board that snowballs in creatures snowballs in cards at the same time. That places it in a green lineage of "creatures-matter" payoffs that turn a tribe or a swarm into a card-advantage motor. The 4/5 frame matters more than it looks: at this cost it is durable enough to survive a turn of removal and ride out a sweeper aimed at smaller things, so a one-card investment actually lives to fire. Narrowness is what the design pays for its power; noncreature spells, activated abilities, and reanimation effects all leave it cold, so it asks you to commit to a creature-dense build rather than splash it as generic advantage. The "may" clause is a quiet kindness, letting you skip the draw when your library is thin or your life total is bleeding from elsewhere, but the assumption baked in is that you will almost always want the card. Land it with no follow-up and it does nothing; land it the turn before a flood of bodies and it threatens to bury an opponent under a refilled hand.