Ethereal Haunt

While you may not have the same magical affinities of the average spellcaster or Arcane Trickster, you have discovered a deeper talent within yourself that reaches out to the Ethereal Plane; the perfect hiding spot. While most would struggle to fare in such a place, you effortlessly slip into the Border Ethereal, blipping in and out of physical space.

Ethereal Eyes

Your learned connection to the Ethereal gives you an edge when seeing and hiding. When you take this archetype at 3rd level, you gain the Truesight passive sense out to 60 feet. Those with Truesight attempting to perceive you while you are hiding must make a Wisdom (Perception) check contested with your Dexterity (Stealth).

Dimensional Severance

As part of your practices with the Ethereal, you learn ways to bypass its limitations, ironically by harnessing them and using this newfound extraplanar power to your advantage. Starting at 3rd level, on every Rogue level you take that wouldn't already increase your Sneak Attack damage, you gain an additional 1d6. All extra damage you add from Sneak Attack now becomes magical Force.

When you hit with a Sneak Attack, the target must succeed on a saving throw using their spellcasting ability (DC = 8 + your proficiency bonus + your INT modifier) or lose the ability to use any magic that would teleport them to another place until the end of their next turn. On a critical hit, the target has disadvantage on their saving throw.

Savage Apparition

As you progressively learn to shift in and out of physicality, you begin to lose your corporeal form, allowing you to shift from shadow to shadow, often appearing in two places at once. Beginning at 6th level, if you attempt a Sneak Attack and miss your target, you can use your reaction to separate your very being into two halves for a brief moment, conjuring a grey, translucent clone of yourself anywhere within 5 feet of the creature. While this clone is active, it is visible both inside and outside the Ethereal, and you can move away from the target without provoking opportunity attacks. Your clone makes one melee attack using a weapon you are wielding at advantage against the same target. If the attack hits, you may choose to turn the damage into magical Force, and the target has disadvantage on the next attack roll it makes before the end of its next turn. The clone then disappears.

Between the Lines

In a breakthrough discovery at level 13, you find that you can open a subtle connection directly to the Ethereal Plane, making it your personal playground for a short time. As an action, you can cast blink to delve into the Border Ethereal, rather than just the Ethereal Plane, for 1 minute. In the Border Ethereal, all objects around you appear grey, and those outside have disadvantage on checks to perceive you.

On the start of every turn you remain there, rather than at the end, roll a d20 to resist being automatically pulled back into your original plane on an 11 or higher. If you are successful, you can Sneak Attack any creature you can see within range, and your attacks are always at advantage if your targets cannot see you, but you always receive damage as if you hadn't left. If you roll a 10 or below, the spell ends and you are pulled back into the plane you were in prior to the spell's casting. You can do this a number of times equal to your Intelligence modifier (a minimum of once) per long rest.

Voices in the Air

At 17th level, your mastery over Ethereal travel becomes profound. You can cast blink as a bonus action on your turn, and the spell's duration is extended to 1 hour. While in the Border Ethereal, you gain resistance to all nonmagical damage, you can designate any number of creatures within 60 feet as allies who can see and interact with you, you have advantage on Dexterity (Stealth) checks to hide from creatures attempting to perceive you, and the roll to stay in this plane becomes a DC 10 Intelligence saving throw instead.

In addition, when a creature within a 60 foot radius of you takes damage while you are in the Ethereal or Border Ethereal, you can use your reaction to bring them into the Border Ethereal while you descend into the plane they were on. Until the end of their next turn, they remain in the Border Ethereal and gain the same benefits you would, before automatically returning to the same plane as you. So long as the duration of the spell has not elapsed since its casting and assuming you have it available, you may use your reaction at any time to return to the Border Ethereal.