If the target of the ability of Tilonalli’s Skinshifter becomes an illegal target, Tilonalli’s Skinshifter won’t become a copy of anything. It will remain a 0/1 attacking creature.

Tilonalli’s Skinshifter copies the printed values of the target creature, plus any copy effects that have been applied to it. It won’t copy counters on that creature or effects that have changed its power, toughness, types, color, or so on.

Notably, Tilonalli’s Skinshifter won’t copy effects that make a noncreature permanent become a creature. If this causes Tilonalli’s Skinshifter to stop being a creature, it’s removed from combat.

If Tilonalli’s Skinshifter copies a creature that’s copying something else, it will become whatever the target is copying.

If an effect from a resolving spell or ability, such as that of Rallying Roar, begins to apply to Tilonalli’s Skinshifter before it becomes a copy of another creature, that effect will continue to apply.

Because attackers have already been declared, any abilities Tilonalli’s Skinshifter copies that trigger when it or other creatures attack won’t trigger.

Once the ability of Tilonalli’s Skinshifter resolves, its new characteristics don’t change if the characteristics of the copied creature change or if the copied creature leaves the battlefield.

Tilonalli’s Skinshifter remains a copy of the creature through the end step. Damage is removed from it at the same time that it stops being a copy.