Corrosive Mentor

Wither was a keyword built to make combat damage stick: instead of marking damage that washes off when the turn ends, a wither creature carves -1/-1 counters into whatever it fights, shrinking the body permanently. This 1/3 lifts that mechanic off the individual attacker and grants it to a whole color. Every black creature you control now fights as though wither were stapled on, which turns ordinary combat into slow disassembly: trades become favorable, multiblocks leave permanent scars, and any black creature that survives a fight has wounded the board rather than merely stalled it. The lord-style grant is the wrinkle. Wither normally lives on a single attacker, paid for on that one body; this converts it into a static blanket, so the payoff scales with how many black creatures you can deploy rather than with the size of any of them. It quietly punishes +1/+1-counter strategies, too: a wither strike eats those counters before it bites into base toughness, peeling buffs off a pumped attacker. The 1/3 frame is honest about the role: this is an enabler, not a threat, a card whose own combat math matters less than the math it imposes on everything else you control. Without a board built to exploit it, it accomplishes almost nothing, and that emptiness is precisely what lets a color-wide keyword grant sit at three mana without warping anything.