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?
Best Answer
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
Flame Strike, PHB p. 242
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:
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.