Shroudbearer

Shroudbearers are individuals that possess a bizarre affinity with the Shadowfell, either by happenstance, ritual, or birthright, being able to channel and act as a living conduit for its essence.

One of the more common manifestations of this is an ability to project fragments of the plane's malign influence unto others.  Upon finding purchase in the flesh of its quarry, whispers cascade in the mind of the wielder: a vague static hiss of muted voices that soon escalate into psychic crescendo, telling him how he might best snuff the life of his foe, regaling its weakness and vulnerabilities, bestowing fever visions of imminent mortality that all but guide the hand of his weapon to strike true.

Some even possess the monstrous ability to take up the soul and guise of their victims, ensuring the true death of their quarry

Death Shroud

As an action or bonus action, you can place a magical curse, known as a shroud, on a creature you can see that's within 120 feet of you, marking it as your shroud target.  A creature may be subject to up to four of your shrouds at a time.  Your shrouds last until you target another creature with this feature, the target dies, or the remove curse spell is cast upon the target at a level at least equal to half your Rogue level.

While your shroud target has four shrouds, you know any damage vulnerabilities it has, and your attacks against it cannot have disadvantage.  Further, when you hit that target with an attack that uses your Sneak Attack feature, you can have that attack critically hit.  If you do, all of you shrouds on the target are expended.

Step From Shadow

At level 3, you can emerge from the very shadow of those marked for death, beckoned by the magic of your shrouds.

As a bonus action, you can magically teleport up to 15 feet for each of your shrouds on your shroud target to an unoccupied space that's adjacent to it.

Dead Ringer

At level 9, you gain the ability to take up the souls of your victims, granting the unsettling ability to assume their likeness.

When you kill a shroud target, you can choose to take its soul (if it has any).  If you do, you can immediately transform your appearance to perfectly duplicate the appearance of that creature as per the Change Appearance effect of the alter self  spell ( no action required).  While its soul is in your possession, that creature can't be revived short of divine intervention or a wish spell.  If you die, you take another soul, or your shroud target is revived, its soul is freed from your possession.

Further, as in action, you can transform your appearance to that of any creature whose soul you possess as per the Change Appearance effect of the alter self spell.

A transformation made in this way lasts until you dismiss it as a bonus action or you transform again.

Reaper's Beckon

At level 13, no quarry can long elude you, the curse laid upon it calling to you incessantly, seeking out its bearer's death.

You always know the direction and approximate distance to your shroud target while it's on the same plane.  You know your shroud target's exact location while it's within 120 feet of you.  You can never be surprised by your shroud target, nor can it become hidden or invisible to you.

Soul Eater

At level 17, the influence of the Shadowfell has changed you into a monstrosity, granting you the ability to wholly consume the souls of those who have fallen before your curse.

Souls are no longer freed from your possession when you take another with your Dead Ringer feature.

Further, you can exploit a soul you possess as per the soul cage spell.  If you do, you cannot do so again for 1 minute.  A soul exploited in this way is utterly consumed: you no longer possess it, and the creature it's from can no longer be revived even if you die, short of divine intervention or a wish spell.