Does Colossus Slayer Crit?

Since you will often be scoring a critical hit with surprise, the question about whether you roll extra damage, like the damage from Colossus Slayer, again may come up. The answer is yes.

What does colossus Slayer do?

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.

Does colossus Slayer stack with Hunter's Mark?

For example, if two clerics cast bless on the same target, that character gains the spell's benefit only once; he or she doesn't get to roll two bonus dice. Since this is the only main rule on stacking, and Colossus Slayer isn't Hunter's Mark, it stacks with Hunter's Mark.