Neerdiv, Devious Diver

The trick is in the trigger word: not attacking, but becoming tapped. That distinction turns the mill from a combat reward into an engine that fires off anything that taps this Merfolk, whether an attack, a convoke cost, a tap-for-value ability, or an opponent's effect that forces it down. Point the mill at your own library and you set up the second clause, which pays out every time you reach back into the graveyard: a card drawn and a counter that grows the mill feeding it. That loop is the whole point. Each +1/+1 counter widens the next tap's mill, which digs deeper into the yard, which surfaces more spells and abilities to cast or activate from below, which draws another card and adds another counter. It is a spellbook engine that generates advantage by recycling the graveyard rather than by hoarding a hand: the draw comes stapled to graveyard-fueled recursion, so the yard is at once the fuel and the reward. The two clauses work as a matched pair, one filling the yard and the other rewarding you for emptying it. The 2/2 body sits inert until you start turning the crank, which is the restraint that keeps a symmetrical mill-any-player effect from collapsing into a plain two-turn clock. Building around it means stocking the graveyard with flashback, jump-start, and disturb remnants, then finding repeatable ways to tap the diver without shipping it into a fight it cannot win.