永劫の不屈

あなたがライフを得るたび、対戦相手１人を対象とする。そのプレイヤーはその点数に等しい点数のライフを失う。
永劫の不屈が死亡したとき、これがクリーチャーであった場合、これをオーナーのコントロール下で戦場に戻す。これはエンチャントである。（これはクリーチャーではない。）

Enduring Tenacity

Whenever you gain life, target opponent loses that much life.
When Enduring Tenacity dies, if it was a creature, return it to the battlefield under its owner's control. It's an enchantment. (It's not a creature.)

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Duskmourn: House of Horror

A token that's a copy of Enduring Tenacity won't return to the battlefield when its last ability resolves.

Each creature with lifelink dealing combat damage causes a separate life-gaining event. For example, if two creatures you control with lifelink deal combat damage at the same time, Enduring Tenacity's first ability will trigger twice and you may choose a different opponent for each trigger. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, planeswalkers, and/or battles at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.

If a nontoken permanent that's a copy of Enduring Tenacity dies while it's a creature, it will return to the battlefield as an enchantment when its last ability resolves. It won't have any card types other than enchantment.

If an ability triggers whenever an opponent loses life and its effect causes you to gain life, such as the ability of Exquisite Blood, this will loop until either you win the game or a player takes an action to break the loop.

If you gain an amount of life "for each" of something, that life is gained as one event and Enduring Tenacity's first ability triggers only once.

In a Two-Headed Giant game, life gained by your teammate won't cause Enduring Tenacity's first ability to trigger, even though it caused your team's life total to increase.

Snake and Glimmer are both creature types. Enduring Tenacity won't have those creature types when its last ability returns it to the battlefield because it won't be a creature.