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.

Umbra armor has no effect if the enchanted creature is put into a graveyard for any other reason, such as if it's sacrificed, if the "legend rule" applies to it, or if its toughness is 0 or less.

If a spell or ability says that it would "destroy" a creature enchanted with an Aura that has umbra armor, that spell or ability is what causes the Aura to be destroyed instead. Umbra armor doesn't destroy the Aura; rather, it changes the effects of the spell or ability. On the other hand, if a spell or ability deals lethal damage to a creature enchanted with an Aura that has umbra armor, the game rules regarding lethal damage cause the Aura to be destroyed, not that spell or ability.

If a spell or ability would destroy both an Aura with umbra armor and the creature it's enchanting at the same time, umbra armor's effect will save the enchanted creature from being destroyed. Instead, the spell or ability will destroy the Aura in two different ways at the same time, but the result is the same as destroying it once.

Attaching Dog Umbra to a creature another player controls that's already attacking or blocking won't remove that creature from combat.

Dog Umbra continuously checks to see whether the controller of the enchanted creature is a different player from the controller of Dog Umbra. If either permanent changes controllers, the effect may change.

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.

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.

If a creature enchanted with an Aura that has umbra armor would be destroyed by multiple state-based actions at the same time (most likely because a creature with deathtouch has dealt damage to that creature greater than or equal to its toughness), umbra armor's effect will replace all of them and save the creature.

If a creature you control is enchanted with multiple Auras that have umbra armor, and the enchanted creature would be destroyed, one of those Auras is destroyed instead—but only one of them. You choose which one because you control the enchanted creature.

Umbra armor's effect is not regeneration. Specifically, if umbra armor's effect is applied, the enchanted creature does not become tapped and is not removed from combat as a result. Effects that say the enchanted creature can't be regenerated won't prevent umbra armor's effect from being applied.