As a warlock with the Fiend pact you gain Dark One's Blessing:
Starting at 1st level, when you reduce a hostile creature
to 0 hit points, you gain temporary hit points equal to your
Charisma modifier + your warlock level (minimum o f 1).
Say you have an item or a spell like Armor of Agathys that damages enemies when they attack you (so auto damage with no save for them and no attack roll for you).
Lets assume you have 1 Temp Hit Point, and get damaged for 5. If that auto damage kills the enemy, and you gain THP from Dark One's Blessing, would you take 4 damage from your real HP or four off the top of the new THP gain?
Best Answer
This one is a little bit more tricky to answer than your previous question about Temp HP from one effect canceling out any others that rely on Temp HP from another effect. (Found here: Dark One's Blessing and Armor of Agathys Synergy? )
So then, you have to think in order of events. Combat happens very quickly (an entire round happens in the span of 6 seconds) and things don't really happen at once when it comes down to the rules. The creature doesn't hit you, take damage, your Armor of Agathys goes away but you gain more temp HP because the Armor of Agathys damage killed it and you reaped it's energy with Dark Ones Blessing.
It more happens like this: A creature attacks you, your Armor of Agathys is holding at 1hp , barely there anymore, 1 damage is eaten up by the barrier and the remaining 4 cleave into your shoulder (Since he broke the barrier with damage remaining we assume teh attack follows through, if only a little bit.) and the creature takes cold damage as a result of the Armor of Agathys doing it's job. It falls over, frozen and dead, and the Dark one blesses you with extra HP. After the creature dies, and after it's attack and damage is resolved.
I hope that answers your question?