Roguish Archetype: Spectral Infiltrator

Kaya's Guile by Jason Rainville

Ghostly Assassin

The shadowed figure dips in and out of sight. The mage who is his target doesn't see what's coming until it's too late, even through her mage sight. When the blade sinks into the mages's flesh it is just a moment of weakness and the mage casts her spells in that location but instead it was useless. The assassin is already gone. Before the mage knew it she was dead and could see the assassin in the ethereal realm. The assassin dips in and out if existence itself in order to strike the next targets.

Fade Out
Starting at 3rd level, when you would have the ability to make a hit with sneak attack, you can forgo the attack roll to instead enter the ethereal realm if the target of the attack would fail a Wisdom save. The DC is determined by [rolling for the damage you would have done with your weapon + rolling for the sneak attack die + your proficiency modifier]. (For example: a dagger could roll a 2 on a d4, and at level 1 a sneak attack die could roll a 6 on your d6, and as well you would have +2 proficiency modifier. Therefore the DC would be 10.) If the creature would succeed on its save it still takes half the potential damage but you may not enter the ethereal realm by this feature until the turn after next. You can exist in the ethereal realm in this way for a number of minutes equal to your proficiency modifier.

Ethereal Strike
As well at 3rd level, while invisible or in the ethereal realm, you have advantage to the first attack roll upon exiting that state.

Shadowed Prey
Beginning at 9th level, creatures that are not fully anchored to the material plane take double your sneak attack damage. Ghosts and all creatures that are able to enter the ethereal realm (whether by abilities or by spell effects) are considered to be partially anchored.

Phantasmal Fury
At 13th level, upon exiting or entering the ethereal realm, you can cast phantasmal killer a number of times equal to your proficiency per long rest.

Veiled Assassin
Starting at 17th level you have learned how to enter the ethereal realm at will and therefore no longer need to forgo your sneak attack damage in order to enter it. There is no longer a limit on the amount of time you can stay in the ethereal realm.