Homebrew 2nd level Ranger spell. Please critique

Slaying Strike

Level 2 Divination (Ranger)

Casting Time: Bonus Action, after hitting a creature with a weapon or unarmed strike

Range: Self

Components: V

Duration: Instantaneous

As you hit a target you have designated as your quarry with your Hunter's Mark, you discern its most vital point and attack it unerringly. You deal an extra 1d8 damage to the target of the same damage type of the attack that precedes this spell. You also turn that attack into a critical hit. Creatures that are immune to critical hits are immune to this spell. 

Is the extra 1d8 damage doubled by the crit, or not? If not, it's a worse Divine Smite. If yes, it's basically exactly as good as Divine Smite. Also, I'm not sure there are any monsters that are immune to crits in 5e.

Personally, I think you should try to rework this into something more unique and exciting. Maybe something like this (these numbers are just the first ones that came to mind, and are probably not balanced): The attack deals an extra 1d8 damage, and if the target has less than 10 HP remaining after taking damage from the attack, it dies.