An Aura controlled by another player does not cause a creature you control to be modified.

A creature that is equipped is considered modified no matter who controls the Equipment that's attached to it.

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.

A creature with a counter on it is considered modified no matter what kind of counter it is or which player put it on that creature.

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.

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.

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.

Lion Umbra can't become attached to a creature unless that creature is already modified.

If a Lion Umbra you control is attached to a creature you control, that creature will continue to be a "modified creature" as long as Lion Umbra stays attached to it, regardless of whether it has any other Auras you control or Equipment attached to it or whether it has counters on it. However, if a Lion Umbra you control is attached to a creature you don't control and that creature has no Auras controlled by that creature's controller attached to it, no Equipment attached to it, and no counters on it, Lion Umbra will be put into its owner's graveyard due to being attached to an illegal object the next time state-based actions are performed.

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.

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.