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