You focus your training on the grim art of death. Those who adhere to this archetype are diverse: hired killers, spies, bounty hunters, and even specially anointed priests trained to exterminate the enemies of their deity. Stealth, poison, and disguise help you eliminate your foes with deadly efficiency. 

This is a updated variant of the Assassin subclass to be used in Homebrew games.

Tricks of the Trade

3rd-level Assassin (update) feature

You gain proficiency with the disguise kit and the poisoner’s kit. Your resourceful efficiency allows you to craft disguises and poisons at half the cost and time required. You have resistance to poison damage due to your time spent building up your tolerance.  

Also, you can use the bonus action granted by your Cunning Action to don a disguise or apply poison to a weapon or piece of ammunition.

Assassinate

3rd-level Assassin (update) feature

You are at your deadliest when you get the drop on your enemies. You gain the following benefits:

• You can add your proficiency bonus to your initiative rolls.
• Your attacks gain advantage against creatures who have not yet taken a turn in a combat encounter or who are suffering from the poisoned condition.
• Your attacks become critical hits when hitting creatures who are surprised.

Infiltration Expertise

9th-level Assassin (update) feature

You gain proficiency with forgery kits. You skill checks involving using the disguise kit, forgery kit, or poisoner's kit have advantage. Items crafted by them also benefit from this advantage when performing skill checks such as Sleight of Hand to avoid detection when poisoning food or drink.

If you spend at least 1 minute observing or interacting with another creature, you can learn certain information about them. You can try to unerringly mimic another person’s speech, writing, and behavior after doing so. Studying behavior has given you a cerebral ability to influence others. You may use your Intelligence modifier instead of your Charisma modifier when you resolve a Charisma-based skill check.

Also, As part of a long rest, you can unfailingly create false identities for yourself. Using both the disguise kit and the forgery kit to create a false identity that can be used to appropriate clothing, letters of introduction, and official-looking certification to establish yourself as a member of a trading house from a remote city so you can insinuate yourself into the company of other wealthy merchants. Thereafter, if you adopt the new identity as a disguise, other creatures believe you to be that person until given an obvious reason not to.

Expert Ambusher

13th-level Assassin (update) feature

You have gain the ability to always get the drop on your enemies and to end them rightly. Whenever you make a initiative roll, treat a roll of 9 or lower on the d20 as an 10. Also, when you have advantage on a weapon attack roll, your weapon attacks score a critical hit on a roll of 19 or 20. 

Death Strike

17-th level Assassin (update) feature

You become a master of instant death. When you attack and hit a creature and deal sneak attack damage, it must make a Constitution saving throw (DC 8 + your Dexterity modifier + your proficiency bonus). On a failed save, double the sneak attack damage against the creature and the creature is stunned until the end of your next turn.