A better assassin made by u/ejaculatingbees on reddit

Bonus Proficiencies

When you choose this archetype at 4rd level, you gain proficiency with the disguise kit and the poisoner's kit. Your proficiency bonus is doubled for any ability check you make that uses either of those proficiencies.

Assassinate

You have advantage on initiative rolls, as well as on attack rolls against any creature that hasn't taken a turn in the combat yet. In addition, any hit you score against a creature that is suprised is a critical hit.

No escape

Whenever you hit a creature with less than half of its hit points remaining with a weapon attack or reduce a creatures hit points below half with a weapon attack, their speed is halved and they lose the ability to take the disengage action until the end of their next turn. At 15th level, you also prevent the creature you hit from teleporting for the same duration.

Know your Mark

You can closely observe a creature for one minute. Doing so allows you to learn their armor class and hit point maximum. You also gain advantage on all Charisma checks you make to influence them if you share a language, as well as all attacks you make against them should you choose to attack them. In addition, all sneak attacks you make against this creature deal an extra 2d6 damage.

Should the creature survive a combat encounter with you, you lose all the benefits of this feature against them unless you are able to observe them again. You can only have one creature marked in this way at a time.

Death Strike

When you attack and hit a creature that is surprised, it must make a Constitution saving throw (DC 8 + your Dexterity modifier + your proficiency bonus). On a failed save, double the damage of your attack against the creature.