Yesterday, my Half-Orc was nearly insta-killed by a nasty crit from a wraith by reducing my maximum hitpoints to 0. Our DM, out of the kindness of his heart, ruled that I could choose to be reduced to 1 hit point instead because of relentless endurance. But I want to know what the rules say.
Does Relentless Endurance interact with reducing maximum hitpoints? Can it stop me from going to 0 hit points maximum as I've done?
Two issues:
From MM p. 302 on the Life Drain skill.
This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.
From PHB p. 41 on Relentless Endurance
When you are reduced to zero hit points but not killed outright, you can drop to 1 hit point.
Technically, you were killed outright since the rule says "the target dies" so you were dead (not just reduced to zero hit points) as a result of the wraith's Life Drain skill. This is a specific case (target dies at 0HP and 0 max HP) rather than the general case of death saves / stabilization chances before dying.
PHB p. 197 on Dropping to 0 Hit Points
When you are reduced to 0 HP you either die outright or fall unconscious //snip// if damage reduces to 0 HP and fails to kill you you fall unconscious.
As written, the Life Drain skill killed you outright.
The other point is that if your max hit points are zero, they can't be 1 since 0 is the maximum value and 1 is greater than that.
All the above said, I like the ruling for its cinematic and heroic overtones: "he kept fighting because he didn't know he was dead" seems a good fit for a half-orc in the heat of battle. (Not to mention that scene in Fatal Attraction where Glenn Close (dead?) lunges up and out of the bathtub to attack one more time).
Relentless Endurance says nothing about hit point maximum; it's all about your current hit point total.
Your GM was nice, and it's an interesting thought. But it's not what the ability says:
Relentless Endurance. When you are reduced to zero hit points but not killed outright, you can drop to one hit point instead. (PHB p.41)
Every time in the corebooks that the phrase "hit point(s)" is used it refers to one's current health status. "Hit point maximum" is always specifically used when referring to the upper limit of one's hit points.
