Is damage from multiple damage types cumulative for death and massive damage?

Question

Scenario 1: A grimlock hits Alice with a Spiked Bone Club, dealing 5 bludgeoning damage, plus 2 piercing damage. Alice's HP was 12.

Using the optional Massive Damage rules on page 273 of the DMG, does this count as more than half the HP from a single source, or less than half, but twice?

Scenario 2: Assume the same grimlock hits Bob with the same attack, this time rolling 6 bludgeoning damage, plus 3 piercing damage. Bob's Max HP is 8, and he had 1 HP remaining before he was hit.

Using the Instant Death rules on page 197 of the PHB, does the character die, or are the two damage types tracked separately? Does the character have to make a death saving throw for the second part of the damage?

Answer 1

Yes.

Damage from the same source with multiple damage types (such as a grimlock's Spiked Bone Club attack, or the flame strike spell) is all inflicted at once. Nothing in the rules for Damage Types (PHB, p. 196) indicates that a given effect is restricted to a single type of damage. The wording for these effects doesn't separate the damage types into different sources:

Spiked Bone Club, MM p. 175

Hit: 5 (1d4 + 3) bludgeoning damage plus 2 (1d4) piercing damage.

Flame Strike, PHB p. 242

A creature takes 4d6 fire damage and 4d6 radiant damage on a failed save ...

If flame strike's damage types were supposed to be delivered by different sources, then they wouldn't be conditional on the same (singular) saving throw. Likewise, the Spiked Bone Club's damage is the effect of the (singular) Hit.

To further support this, the optional Vitality rules (presented in this Unearthed Arcana article) use this language:

Whenever a character takes 10 or more damage from an attack or effect, the character loses vitality.

This makes it clear that a single attack's total damage, regardless of its constituent types, is what matters. The Massive Damage and Instant Death rules have the same flavor and can be construed to work in the same way.

Of course, resistances and vulnerabilities may apply separately to each damage type that an attack inflicts. A monster that is resistant to fire but vulnerable to radiant damage that fails its save versus a flame strike will take an average of (14 / 2) + (14 * 2) = 35 damage from the spell as a single damage source.

Answer 2

Yes.

The instant death/death check rules are not as legalistically clear as one might like, but the intent is that they count per attack. The grimlock's club is one attack even though it delivers multiple dice/types of damage, so the total is what is applied to the Instant Death rule.

See under "Damage,"

Each weapon, spell, and harmful monster ability specifies the damage it deals. You roll the damage die or dice, add any modifiers, and apply the damage to your target.

While the instant death rule itself just says "damage remaining," where you might try to interpret that as "per die" or "per part of the damage," the example right below it says "if she takes 18 points of damage from an attack..." (emphasis mine). Therefore any damage from that attack - multiple types, energy stacked on, etc. - is all one big chunk of damage for purposes of these rules.

In your Scenario 1 that's simply 7 total points of damage from an attack and in Scenario 2 it's 9 total points of damage from an attack.