Swimmer in Nightmares
A 1/4 body sits as a wall until a graveyard fills, and then it stops being one. Ten or more cards in a single graveyard turn this into a static 4/4, and the wording is the whole point: it counts one yard, not the combined total, so the size hinges on whether your own self-milling engine pushes past ten or an opponent grinds themselves down to fuel it either way. That makes it a beater on a shared clock, at home in decks built to dump cards fast rather than ones that wait for a long game to age into the threshold. The second line names the deck outright. Unblockable while you control an Ashiok planeswalker is a deliberately narrow key: it chains the creature's evasion to a specific planeswalker whose identity is exiling cards and swelling graveyards, so both halves point at the same shell. Ashiok mills, the yard climbs past ten, the Swimmer stands up as a 4/4 that can't be blocked, and a defensive statline becomes a repeatable four-damage clock. Pulled out of that build it stays a durable blocker with upside; inside the mill deck it was drawn for, it is the finisher that cashes in what the mill sets up. The balancing act is that neither half moves without commitment: the +3/+0 asks for a full graveyard, and the evasion asks you to run the exact planeswalker that unlocks it.
