Treefolk Umbra

Treefolk Umbra's effect doesn't actually change the enchanted creature's power. It changes only the amount of combat damage the creature assigns. All other rules and effects that check power or toughness use the real values. For example, Savage Swipe won't cause the enchanted creature to fight with its toughness.

Umbra armor's effect is applied no matter why the enchanted creature would be destroyed: because it's been dealt lethal damage, or because it's being affected by an effect that says to "destroy" it (such as Breathe Your Last). In either case, all damage is removed from the creature and the Aura is destroyed instead.

Umbra armor's effect is mandatory. If the enchanted creature would be destroyed, you must remove all damage from it (if it has any) and destroy the Aura that has umbra armor instead.