Assassin Revised

This is basically just a revised version of the 5e Assassin Subclass.

Tools of the Trade

Starting at 3rd level, you gain proficiency with the Disguise Kit and the Poisoner's Kit, and you can craft poisons using your Poisoner's Kit at a rate of one hour per 5GP of work instead of the usual one day per 5GP. In addition, you can use your Cunning Action feature to apply poison to a weapon or piece of ammunition as a bonus action.

Assassinate

When you choose this archetype at 3rd level, you have advantage on attack rolls against any creature that hasn't taken a turn in combat yet and you add half of your levels in the Rogue Class (rounded down) to your Initiative Rolls. Additionally, any hit you score against a creature that is surprised is a critical hit.

Lethality

Starting at 9th level, when you attack and hit a creature that is surprised, you increase your Sneak Attack damage dice by 2d6. Additionally, any creature you have surprised has disadvantage on the first saving throw against any poison you have applied to the weapon or piece of ammunition used for the Sneak Attack.

Master of Poisons

At 13th level, when you apply poison to a weapon or piece of ammunition and subsequently deal damage, it ignores resistance to poison damage and treats immunity to poison damage as resistance to poison damage. Additionally, thanks to your extensive experience testing your own poisons, you gain advantage on saving throws against poison, and you have resistance against poison damage.

Death Strike

When you reach 17th level, 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.