Life-Thief

Often called "nosferati" after the vampire who taught the first of their kind, Life-Thieves are rogues with the profane power to slake their thirst for immortality.  Able to heal their wounds and kill at a glance, they are widely feared.  Most are evil, although a handful swear to prey only upon the wicked.

Steal Life

Starting when you choose this archetype at 3rd level, you learn how to take others' life force for your own.  When you deal sneak attack damage, the target takes additional necrotic damage equal to the number of sneak attack dice you roll.  You gain temporary hit points equal to the necrotic damage suffered.  If the attack kills the target or reduces it to 0 hit points, you may instead regain hit points equal to the necrotic damage suffered.

All features from this archetype cannot give you more temporary hit points or restore your hit points more than the hit points the target has, including those required to slay it outright.  For example, imagine a rat with 1 hit point, its maximum.  Taking 2 points of damage would kill it outright.  If you are 5th level and sneak attack this rat with 1 hit point, you could regain only 2 hit points with this feature, even though the attack deals an additional 3 necrotic damage.

Delay Death

At 3rd level, for each sentient creature you kill, you do not age for the next 24 hours.

Devour the Weak

At 9th level, you can snuff out the essence of feeble creatures with hardly a thought.  You automatically sense the location of all creatures within 30 feet of you whose current hit points are equal to or under your Rogue level. 

As a bonus action, you can choose one creature you sense with this feature.  It takes necrotic damage equal to your Rogue level, and you regain hit points equal to the necrotic damage it suffers as you visibly extract the life from it.  If the target is at 0 hit points, it takes no damage, but immediately dies.

Drain Resistance

Beginning at 9th level, you have resistance to necrotic damage, and your hit point maximum can't be reduced.  You are sufficiently skilled at manipulating life essence to protect your own from being siphoned.

Life-Robbing Aura

At 13th level, whenever a creature within 30 feet of you dies, is reduced to 0 hit points, or starts its turn at 0 hit points, you can immediately harvest its departing soul or remaining vitality.  If you do so, the target dies if it is still alive.  Regardless, you have advantage on any one attack roll, ability check, or saving throw you make before the end of your next turn.

In addition, your features from this archetype ignore resistance to necrotic damage and treat creatures immune to necrotic as if they instead had resistance to it.

Feast of the Damned

At 17th level, you can truly drain life from other creatures.  This can take one of two forms.

• When you hit with a weapon attack, you can deal additional necrotic damage equal to twice your Rogue level.  If you do so, you regain hit points equal to the necrotic damage suffered as the weapon erupts with dark power.
• As an action, you can use your Devour the Weak feature with any number of creatures.

Once you use this feature, you can't use it again until you finish a long rest.