Question cropped up during our session tonight. As the title suggests, I’m playing a ranger with the Colossus Slayer ability (bonus 1d8 damage when damaging a target below its max HP). Currently at level 6, so I have extra attack, but in the past I’ve only applied the bonus damage once per turn.

Ordinarily, I’m focused on archery, usually staying in the periphery and sniping targets of opportunity, but in a trip to the Underdark, we were ambushed by a Hook Horror, and I ended up attacking with my Longsword. I successfully hit my first attack, and then again on my extra attack. Do I still apply Colossus Slayer in this situation, since it is still the same action? In this case, my DM allowed it, since it was only 1 extra damage, but we really weren’t sure what the “official” ruling is, since the PHB doesn’t really specify. Clarity anyone has would be appreciated!