Does the Way of the Long Death monk's Mastery of Death feature protect against the Disintegrate spell?

Question

Some damaging spells, like disintegrate, have some additional effects when they reduce you to 0 hit points (in this case, reducing the target to a pile of dust).

The Way of the Long Death monk's 11th-level feature, Mastery of Death, states this (SCAG, p. 131):

When you are reduced to 0 hit points, you can expend 1 ki point (no action required) to have 1 hit point instead.

If a level 11 Long Death monk with few hit points left fails the saving throw against disintegrate (doing enough damage to normally reduce him to 0 HP) and attempts to use his Mastery of Death feature, what happens?

1. The disintegration happens before the feature can take effect, so the monk is now dust
2. The Mastery of Death effect happens before the dusting occurs, so the monk is safe (and alive at 1 hit point)

Answer 1

Yes, it prevents you from dying.

From Jeremy Crawford:

Q: Do Mastery of Death or Strength of the Grave function like Death Ward or like Relentless Endurance when it comes to being Disintegrated? I.E. Can they be used after being reduced to 0 or are you disintegrated?

A: Mastery of Death and Strength of the Grave can be used when you drop to 0 hit points. They don't care how you got to 0.

This is in comparison to effects like the half-orc's Relentless Endurance trait that have a special caveat:

When you are reduced to 0 hit points but not killed outright, you can drop to 1 hit point instead.

Which has been addressed in a published Rules Answer by WotC:

Q: If the damage from disintegrate reduces a half-orc to 0 hit points, can Relentless Endurance prevent the orc from turning to ash?

A: If disintegrate reduces you to 0 hit points, you’re killed outright, as you turn to dust. If you’re a half-orc, Relentless Endurance can’t save you.

So, yes... unlike Relentless Endurance, Mastery of Death can save you from Instant Death effects like Disintegrate or Massive Damage.

Answer 2

The monk is not disintegrated because they never reach 0 hit points

The key word in the ability that enables it to protect from disintegration is "instead". This word indicates that the ability replaces one event (dropping to zero HP) with another (going to 1 HP). Since the monk never reached zero HP, the disintegrate spell's "drop to zero HP" clause never comes into play. This is a bit confusing, because the ability triggers on a specific event and then prevents the triggering event from ever happening. But that's fine. Once the ability is triggered, it can't be "un-triggered" even if the triggering event is undone.