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 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 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.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Christopher Moeller
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
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 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.
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 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 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.
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.
Some printings of this card refer to the ability "totem armor". This ability's name has been replaced with "umbra armor". This change does not affect the gameplay function of this card.
Indrik Umbra does not give any creature the ability to block the enchanted creature. It just forces those creatures which are already able to block that creature to do so.
If, during a player's declare blockers step, a creature is tapped or it's affected by a spell or ability that says it can't block, then it doesn't block. If there's a cost associated with having a creature block, its controller isn't forced to pay that cost. The creature doesn't have to block.
