What happens when a creature's toughness is reduced to zero by a -1/-1 effect while it has damage marked on it?

A creature dies (is put into the graveyard) whenever it has toughness 0 or less, or whenever it has damage marked on it equal to or greater than its toughness. These are two separate state-based actions checked simultaneously, but if toughness drops to 0 or less, the creature dies regardless of any damage on it.

Under CR 704.5f, a creature with toughness 0 or less is put into the graveyard as a state-based action. Under CR 704.5g, a creature with lethal damage (damage >= toughness) is also put into the graveyard. Both checks happen at the same time (CR 704.3), but when toughness is 0, the creature is simply gone — the marked damage contributed nothing extra.

Importantly, marked damage on a creature doesn't change its toughness — damage and toughness are tracked separately (CR 120.3). A -1/-1 effect reduces the creature's base or modified toughness directly, which can push it to 0 or below and trigger 704.5f.

Concrete example: You control a 2/2 creature with 1 damage already marked on it. Your opponent casts a spell that gives it -2/-2 until end of turn, making it a 0/0. State-based actions are checked: the creature has toughness 0 (704.5f), so it dies immediately. The previously marked 1 damage is irrelevant — the 0 toughness alone was sufficient to destroy it.

The practical takeaway: reducing toughness to 0 is a clean kill on its own. You don't need to combine it with damage, and any existing damage on the creature doesn't interact with or change this outcome.