Freelance Muscle

The trick is that it borrows size from your biggest creature and stacks it on top of its own 4/4 frame. On the declare-attackers or declare-blockers step, the trigger performs one flat check of the board and takes the greatest power or toughness among your other creatures as X, then hands the attacker a +X/+X that lasts until end of turn. See a 10/10 and it swings as a 14/14, holding that size through the post-combat main phase before sliding back to 4/4 at cleanup. The self-exclusion is deliberate: counting only other creatures keeps the ability from feeding on its own boosted number, so the check stays a single pass. It rewards a top-heavy board over a wide one, because it wants one large creature to read off of, and it will take toughness when toughness is the larger figure, so a defensive wall you never planned to attack with still fuels the swing. Green has a long habit of building creatures that measure the rest of your team and lean on its largest piece instead of carrying a late-game statline of their own; this one takes that idea and applies the borrowed number only in combat. The cost of that bulk is fragility of the source. Because the trigger reads the board at resolution, removal aimed at your biggest creature in response to the trigger shrinks the number before the buff lands, so X reflects only the largest survivor among your remaining creatures. Even two copies attacking together don't reflect each other symmetrically: their triggers resolve one at a time, the first reading a 4/4 into an 8/8, the second reading that new 8/8 into a 12/12.