The Assassin Roguish Archetype was originally published in the Player's Handbook, and is widely regarded as one of the more underwhelming subclasses, as it fails to deliver on its fantasy. The Alternate Assassin is an attempt to bring the Assassin up to par with the other official rogue subclasses.

This subclass was created by laserllama, and the original PDF can be found on GM Binder.

Assassin's Strike

Starting at 3rd level, you are at your deadliest when you get the drop on your enemies. You have advantage 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 surprised is a critical hit, and when you score a critical hit and roll a 1 on any of your damage dice for that attack, you can re-roll those dice. You must use the new roll, even if you roll another 1.

Infiltrator

You have the skills to infiltrate and sow death wherever your target dwells. When you adopt this Archetype at 3rd level, you gain proficiency with the disguise kit and the poisoner's kit.

Moreover, when you make a Charisma (Deception) check while wearing a disguise you created with your disguise kit, you can treat a roll of 7 or lower on the d20 as an 8.

Stolen Identity

You have mastered the dark art of stealing another creature's identity. Starting at 9th level, over the course of an hour, you can craft a perfect disguise of a humanoid creature you have slain, so long as you have the corpse and a disguise kit. To see through your disguise, a creature must succeed on an Intelligence (Investigation) or a Wisdom (Insight) check, contested by your Charisma (Deception) check.

Also, over an 8 hour period, which can be during a long rest, you can use this disguise to completely assume the life of the slain humanoid. After which, other creatures believe you to be that person until given an obvious reason not to.

Impostor

The skill with which you adopt another creautre's identity is nearly supernatural. Beginning at 13th level, when you make a Charisma (Deception) check to thwart a creature trying to see through your disguise, you have advantage on your roll.

Poisoned Blade

Your skills with a poisoner's kit have grown beyond normal limits. At 13th level, you can use a bonus action to soak your blade with poison. The next time you hit with an attack with that blade, you can apply your Sneak Attack bonus, even if you don't meet the normal requirements for Sneak Attack, and your Sneak Attack damage becomes necrotic damage.

Once you use this bonus action you must finish a short or long rest before you can use it again.

Death Strike

You have become a master of instant death. Starting at 17th level, when you hit a creature with a Sneak Attack, you can cause the attack to become an automatic critical hit.

Once you use this feature you must finish a short or long rest before you can use it again.