2019-08-23 : Because damage remains marked on a creature until it’s removed as the turn ends, nonlethal damage dealt to a creature the target player controls may become lethal once its base toughness becomes 1.

2019-08-23 : Effects that raise or lower a creature’s power and/or toughness, such as the effect of Titanic Growth, will apply to that creature no matter when they started to take effect. The same is true for any counters that change its power and/or toughness.

2019-08-23 : In a multiplayer game, if you leave the game after Mass Diminish resolves but before your next turn begins, its effect lasts until your next turn would have begun. It neither expires immediately nor lasts indefinitely.

2019-08-23 : Mass Diminish affects only creatures the target player controls at the time it resolves. Creatures they begin to control later before your next turn won’t be 1/1.

2019-08-23 : Mass Diminish overwrites all previous effects that set a creature’s base power and toughness to specific values. Any power- or toughness-setting effects that start to apply after that ability resolves will overwrite this effect.

2019-08-23 : Mass Diminish targets the player, not their creatures. A creature with hexproof or protection from blue may be shrunk this way.