Eland Umbra

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.

If a creature enchanted with an Aura that has umbra armor has indestructible, lethal damage and effects that try to destroy it simply have no effect. Umbra armor won't do anything because it won't have to.

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.