Enduring Passage allows Scarlett to travel in the Twilight World for longer time periods; I.e., it extends the duration of The Passage. There are four Levels of Mastery, each level increasing the duration of The Passage still further (the original duration of The Passage is 15 seconds.).
Level 1 - Extends duration by 5 seconds.
Level 2 - Extends duration by 8 seconds.
Level 3 - Extends duration by 12 seconds.
Level 4 - Extends duration by 20 seconds.
Scarlett will find it useful to extend her time in the Twilight World if she wishes to evade multiple enemies. It is extremely useful when fighting Raging Ghosts, which can only be harmed in the Twilight World. At the fourth Level of Mastery, the duration of The Passage will exceed its Cooldown time, which means Scarlett will be able to re-invoke The Passage as soon as it expires (as long as she has sufficient Mental Energy.)
Scarlett can only learn the Enduring Passage Mental Skill after she has attained at least one Level of Mastery with its prerequisite, Wasted Years; after she has accumulated sufficient Skill Points; and after she finds a Necromancy Trainer (possibly Aeris in the Inner City). Scarlett can increase her Level of Mastery of this skill by spending additional Skill Points with any capable Necromancy Trainer.