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.

The point of the spell is to give players the Slayer role of the Ranger a few times a day, specifically at. Tier 2. The 1d8 damage is doubled by the autocrit. Any other dice are doubled as well. This includes the 1d6 from the Hunter's Mark, and the weapon die. It triggers the additional die of damage on Silvered Weapons. It doubles dice from Vicious weapons, It also doubles the dice from Giant Slayer and Dragon Slayer weapons, as well as those from Flametongues and Frost Brands.

It's poorly balanced since the amount of damage the spell does can vary from 2d8 to >10d6 depending on the character build using it (i.e. a Ranger-Rogue multiclass with a magic weapon), for the same limited resource cost. A second level spell should dealing ~2d8-3d6 damage. 

As Manuntoryx mentioned the language is confusing as well, and it's not very interesting.