A creature dealt lethal damage during combat won’t survive long enough to become a copy of Permeating Mass. If Permeating Mass is dealt lethal damage during combat, the creature Permeating Mass dealt damage to still becomes a copy of it.

A creature with 3 or more damage marked on it that becomes a copy of Permeating Mass will be destroyed immediately after becoming a copy.

The damaged creature copies the printed values of Permeating Mass. It won’t copy counters on Permeating Mass or effects that have changed its power, toughness, types, color, or so on. Notably, this means that a creature that becomes a copy of Permeating Mass has the ability to permeate other creatures as well.

If Permeating Mass becomes a copy of another creature before its triggered ability resolves, the damaged creature will become a copy of that creature.

Effects that have already applied to the damaged creature will continue to apply to it. For example, if Giant Growth had given it +3/+3 earlier in the turn, it will be 4/6.