A Solar's Slaying Longbow attack is unique in that it has the ability to instantly kill an opponent who has 100 hit points or less if they fail a constitution saving throw:
Slaying Longbow. Ranged Weapon Attack: +13 to hit, range 150/600 ft., one target. Hit: 15 (2d8 + 6) piercing damage plus 27 (6d8) radiant damage. If the target is a creature that has 100 hit points or fewer, it must succeed on a DC 15 Constitution saving throw or die.
Assuming the attack hits, are you supposed to roll and apply the weapon damage damage, and then check if the creature meets the hit point threshold? Or, do you check if the creature meets the hit point threshold and then apply the weapon damage afterward?