Colossus Slayer. Your tenacity can wear down
the most potent foes. When you hit a creature with a
weapon attack, the creature takes an extra 1d8 damage
if it's below its hit point maximum. You can deal this
extra damage only once per turn.
For which moment the target must be "below its hit point maximum"? Should I consider its HP before the hit that actually triggers the Colossus Slayer feature? Or can it be the same hit that lowers the target's HP and triggers the feature?
Best Answer
It depends on HP before the hit.
Essentially, an attack can be broken down into a couple phases. First, you select a target. Then you make an attack roll, and determine whether the attack hits or misses (or crits). Then you resolve damage and any other effects (mostly these only happen on a hit).
The damage effectively happens all at once (unless a particular effect explicitly says its damage happens later), so when you're checking whether to add the extra damage for Colossus Slayer, the damage from your hit hasn't been applied yet; Colossus Slayer damage happens at the same time as the rest of the damage for the hit.
The other way of thinking about this is that if Colossus Slayer depended on HP after the hit, it would pretty much always deal its extra damage, unless your hit managed to do zero damage. At which point, they might as well change the feature to read, "You deal an extra 1d8 damage when you hit with an attack." Since the feature isn't phrased that way, it seems unlikely that that's how it's intended to work.