Pako, Arcane Retriever

The trigger fires on the declare-attackers step, not on damage: every swing exiles the top card of each player's library and drops a fetch counter on each, whether or not the attack ever connects. That distinction matters, because the pile compounds turn over turn regardless of blocks, chumps, or evasion. Here is the leash. The exiled cards belong to no one until the other half of the pact arrives; Haldan, Avid Arcanist is the hand that spends the accumulated pile, so on its own this is a growing body that strips the top of every deck at the table and banks a resource it cannot cash. The +1/+1 accrual per noncreature card exiled means the body scales fastest into spell-dense decks, so the design behaves as an engine you plug the whole group into rather than a lone threat you point at one opponent. Haste is doing quiet work: this wants to attack the turn it lands, before removal assembles, because each swing compounds both the counters and the exile bank. It is a scaling design rather than a duel-broken one; head-to-head, the trigger still exiles two cards a turn and grows the body by whatever noncreature cards come off the top. One catch shapes the whole plan: the fetch counters ride your own library too, so until Haldan turns up, each attack peels the top of your deck alongside everyone else's, feeding a stockpile only the partner can eventually draw from.