Can the Grave Domain Cleric's "Sentinel at deaths door" cancel the autocrit from hitting an unconscious person?

Question

The Grave Domain Cleric's Sentinel at Death's Door class feature has the following wording:

At 6th level, you gain the ability to impede death’s progress. As a reaction when you or an ally that you can see within 30 feet of you suffers a critical hit, you can turn that attack into a normal hit. Any effects triggered by a critical hit are canceled.

PHB pg 292 describes the unconscious condition:

• An unconscious creature is incapacitated (see the condition), can’t move or speak, and is unaware of its surroundings
• The creature drops whatever it’s holding and falls prone.
• The creature automatically fails Strength and Dexterity saving throws.
• Attack rolls against the creature have advantage.
• Any attack that hits the creature is a critical hit if the attacker is within 5 feet of the creature.

Does this mean that, RAW, a grave cleric could make it so a pc would only lose one death save from the hit, instead of two?

Answer 1

Yes, the PC protected by Sentinel at Death's Door will take a single death saving throw failure

It's pretty straightforward if we look at all the pieces in sequence, and most of the citations are already in the question.

A creature at 0 hit points is, generally, subject to the Unconscious condition, which transforms any hit into a critical as long as the attacker is within 5 feet:

Any attack that hits the creature is a critical hit if the attacker is within 5 feet of the creature. (PHB, Appendix A: Conditions, Unconscious)

When it applies, the Sentinel at Death's Door feature transforms a critical hit into a normal hit:

As a reaction when you or an ally that you can see within 30 feet of you suffers a critical hit, you can turn that attack into a normal hit. Any effects triggered by a critical hit are canceled. (Xanathar's Guide to Everything, Chapter 1: Subclasses, Cleric, Grave Domain, Grave Domain Features)

So a critical hit becomes a normal hit when this feature is activated. The remaining question is what happens on a regular hit to a creature at 0 HP:

If you take any damage while you have 0 hit points, you suffer a death saving throw failure. If the damage is from a critical hit, you suffer two failures instead. If the damage equals or exceeds your hit point maximum, you suffer instant death. (PHB, Chapter 9: Combat, Damage and Healing, Dropping to 0 Hit Points, Death Saving Throws, Damage at 0 Hit Points)

So in summary:

• The two death saving throw failures are a result of the creature at 0 HP taking a critical hit, but only one failure would be imposed on a non-critical hit
• A critical hit is automatic (under some circumstances) for an unconscious creature target, but this class feature converts a critical hit to a normal hit
• And so if the feature is used then the critical hit becomes a normal hit, and the creature at 0 HP suffers the effects of the normal hit
• That effect is a single, automatic failure on a death saving throw

Answer 2

Yes, this will cancel the auto-crit.

Specific rules take precedence over general ones, and Sentinel at Death's Door, being a class feature, is more specific than the rules for death-saving throws.

Sentinel at Death's Door specifies that it can be activated "when you or an ally that you can see within 30 feet of you suffers a critical hit" (emphasis mine), meaning that its activation is based on when the critical hit is endured by the target, not by a critical hit being rolled.

The loss of two death saves to a critical hit is an effect caused by the critical hit, and therefore, that effect will be cancelled.