Phantom Agent

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Phase

When you take this archetype at 3rd level, you can become intangible to slip through enemies.  You can use a bonus action to make yourself and everything you're carrying or wearing semitransparent and partially incorporeal until the end of your turn.  While incorporeal, you can move through other creatures, objects, and structures, though their attacks and spells will still affect you. For every 5 feet you move through a creature or object, you must spend 10 feet of movement.  If you end your turn inside an object, you are shunted into the nearest unoccupied space and take force damage equal to twice the number of feet you moved.

Ghostly Blade

Starting at 9th Level, when you inflict sneak attack damage, with a melee attack while phased, you can deal necrotic, force or psychic damage instead of the attack's normal type.  The target must succeed on a Wisdom Saving throw (DC=8 + your proficiency bonus + your charisma modifier) or be frightened of you until the end of it's next turn.  It has disadvantage on this saving throw if it did not see you until you attacked it.

Phantom Thief

Beginning at 13th level, you learn magics that help you move in a deadly quiet.  You learn Silence, and cast it a number of times per long rest equal to your Charisma modifier.  You are also always under the effect of nondetection.

Possession

Starting at 17th level, you can steal another creature's will outright.  When you deal necrotic, force, or psychic damage, you can force the target to make a Charisma saving throw DC= 8 + your proficiency bonus + your Charisma saving throw).  Creatures that cannot be charmed or frightened automatically succeed, and a frightened creature has disadvantage on the save.

On a failed save, you possess the creature and enter its body.  You determine its actions and movement on each of its turns.  It takes only the actions you choose, and doesn't do anything that you don't allow it to do.  During this time you can also cause it to use a reaction, but this requires you to use your own reaction as well.  In this state you have full cover, whenever it moves.  You can end this effect at any time, requiring no action.

You have total cover inside the creature, but whenever the possessed creature takes damage, you take half that amount.  you must use your action each turn to maintain control over the creature, and each time it takes damage it makes the Charisma saving throw again, ending the possession on a success.  The possession ends when the creature dies.  You can successfully possess a creature once per long rest.