Base Class: Rogue
Bonus Proficiencies
When you choose this archetype at 3rd level, you gain proficiency with the disguise kit and the poisoner's kit.
Assassinate
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.
Infiltration Expertise
Starting at 9th level, you can unfailingly create false identities for yourself. You must spend seven days and 25 gp to establish the history, profession, and affiliations for an identity. You can't establish an identity that belongs to someone else. For example, you might acquire 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.
Impostor
At 13th level, you gain the ability to unerringly mimic another person's speech, writing, and behavior. You must spend at least three hours studying these three components of the person's behavior, listening to speech, examining handwriting, and observing mannerisms.
Your ruse is indiscernible to the casual observer. If a wary creature suspects something is amiss, you have advantage on any Charisma (Deception) check you make to avoid detection.
Death Strike
Starting at 17th level, you become a master of instant death. 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.
Assassinate
With the Assassinate ability, the Assassin gets advantage on attack rolls against any creature that hasn’t taken a turn in the combat yet. Plus, any hits you score against a creature that is surprised are automatically considered to be a critical hit.
A surprised creature will not have had the chance to act in combat yet. Against a surprised creature, you roll your attack with advantage and get an automatic critical hit if the attack lands.
Infiltration Expertise
With this, you are able to create a new identity. This can be very useful for infiltrating certain organizations and getting closer to targets.
After a week of work (and 25gp), you have a new identity complete with an established history, profession, and affiliations. While you can’t establish an identity that belongs to someone else, your new identity is so well-crafted and convincing that others will believe you to be that person unless you give them an obvious reason not to.
As one would expect, this works best in campaigns with lots of intrigue. Having to take a week for all of the preparation and work necessary to create the identity may not vibe in certain situations. Your party will need to be ok with taking a week to do other things while you create the identity.
Furthermore, if you are needing the identity to infiltrate an event that is in less than a week, you won’t have enough time to prepare.
Impostor
The Assassin’s level 13 ability adds to their effectiveness in all things espionage.
By spending at least three hours studying a person’s behavior, the Assassin can perfectly mimic that person. This includes their speech, behavior, and handwriting as long as all three have been observed.
The casual observer will not be able to tell a difference between you and who you are impersonating. Someone who suspects something may look closer, but you still get advantage on any Charisma (Deception) check you make to avoid being detected.
As with the Infiltration Expertise ability, this works best if you coordinate with your party. Being able to observe a guard captain, for example, gives you the information you need to impersonate them. Assassinating them and taking their identity as cover would give you free reign to explore and provide valuable recon information to your party.
Nevertheless, this ability relies on your party agreeing to a plan that makes use of your skill. If they would prefer to storm the base instead of just sitting there while you do recon work, it may be better to stick with the majority to avoid frustrating other players.
Death Strike
With this ability, a surprised creature that the Assassin hits with an attack must make a Constitution saving throw (DC 8 + your Dexterity modifier + your proficiency bonus). If the creature fails this saving throw, the attack’s damage is doubled.
Generally speaking, a level 17 Rogue will have a Dexterity score of 20 and a proficiency bonus of +6. This means that the DC is 19 unless you have any magic items that would make it more difficult.
So because the creature is surprised, you are attacking with advantage. Your Assassinate ability means that the attack will automatically result in a critical hit, which doubles the damage dice. If the target fails their save, the damage doubles again from Death Strike.
So at level 17, you’re looking at:
(Crit Weapon Damage x2 + Dexterity modifier + Crit Sneak Attack x2) x 2