Umbra Mystic

If a spell or ability says that it would “destroy” a permanent enchanted with an Aura that has totem armor, that spell or ability causes the Aura to be destroyed instead. (This matters for cards such as Karmic Justice.) Totem 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 totem armor, the game rules regarding lethal damage cause the Aura to be destroyed, not that spell or ability.
If a spell or ability (such as Planar Cleansing) would destroy both an Aura with totem armor and the permanent it’s enchanting at the same time, totem armor’s effect will save the enchanted permanent 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.
If a permanent you control is enchanted with multiple Auras that have totem armor, and the enchanted permanent would be destroyed, one of those Auras is destroyed instead — but only one of them. You choose which one because you control the enchanted permanent.
Totem armor’s effect is not regeneration. Specifically, if totem armor’s effect is applied, the enchanted permanent does not become tapped and is not removed from combat as a result. Effects that say the enchanted permanent can’t be regenerated (as Vendetta does) won’t prevent totem armor’s effect from being applied.
Totem armor’s effect is mandatory. If the enchanted permanent would be destroyed, you must remove all damage from it and destroy the Aura that has totem armor instead.
Say you control a permanent that’s enchanted by an Aura you control, and that Aura is itself enchanted by an Aura. If the permanent would be destroyed, instead the first Aura is destroyed... but since that Aura would be destroyed, instead the second Aura is destroyed.
If a creature enchanted with an Aura that has totem armor would be destroyed by multiple state-based actions at the same time the totem armor’s effect will replace all of them and save the creature.
If a permanent you control would be destroyed, and it’s enchanted by an Aura with totem armor, it doesn’t matter who controls that Aura. The totem armor effect is mandatory. If that permanent is enchanted by multiple Auras, you choose which one is destroyed, regardless of who controls them.
Totem armor’s effect is applied no matter why the enchanted permanent 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 Doom Blade). In either case, all damage is removed from the permanent and the Aura is destroyed instead.
All _Rise of the Eldrazi_ cards printed with totem armor are Auras with “enchant creature.” But Umbra Mystic grants totem armor to Auras attached to any permanent you control, not just creatures. The ability works the same way even if the Aura is enchanting a land, an artifact, or any other permanent.
Umbra Mystic grants totem armor to Auras attached to permanents you control, regardless of who controls those Auras. Conversely, it doesn’t grant totem armor to Auras you control that are attached to permanents controlled by other players.
Multiple instances of totem armor on the same Aura are redundant.
Say you control a permanent enchanted with an Aura that has totem armor, and the enchanted permanent has gained a regeneration shield. The next time it would be destroyed, you choose whether to apply the regeneration effect or the totem armor effect. The other effect is unused and remains, in case the permanent would be destroyed again.
Say you control a permanent enchanted with an Aura that has totem armor, and that Aura has gained a regeneration shield. The next time the enchanted permanent would be destroyed, the Aura would be destroyed instead — but it regenerates, so nothing is destroyed at all. Alternately, if that Aura somehow gains indestructible, the enchanted permanent is effectively indestructible as well.
Totem armor has no effect if the enchanted permanent is put into a graveyard for any other reason, such as if it’s sacrificed, if it’s legendary and another legendary permanent with the same name is controlled by the same player, or if its toughness is 0 or less.
If a creature enchanted with an Aura that has totem armor has indestructible, lethal damage and effects that try to destroy it simply have no effect. Totem armor won’t do anything because it won’t have to.