Credit also goes to Ishten and Blackstork: DW + SUBTERFUGE + POISON+ TEMPEST
RECOMMENDED PARTY: 1 Aggro Warrior (Cass) + 2 Barrier-Build Mages. These are essential to keeping you alive.
SKILLS:
1. Stealth
2. Flask of Fire
3. Flask of Frost
4. Flask of Lightning
5. Shadow Strike
6. Flank Attack
7. Twin Fangs/Knockout Powder in Subterfuge for CC (Blackstork)
8. Poisoned Weapons
This build is focused primarily on the Subterfuge aspects, incorporating Poison based skills for additional damage over time. The reason for the continuous poison damage is because, on Nightmare, without proper Aggro and Barrier, DW Rogues can be very squishy and are susceptible to AoE activations from enemy attacks as well as swings from 2H Enemies. The Poison abilities are used to damage opponents as you reposition in Stealth.
To start, get upgraded Flank Attack and Twin Fangs. Skirmisher is essential to place you in Stealth after using Flank Attack. Follow the Twin Fangs side of the Passive Upgrades, getting Dance of Death to restore your Stamina and use your quick-cooldown Abilities. Most importantly, however, these abilities lead to Deathblow, which grants ~300% DMG and removes cooldown on it to use it again.
In Subterfuge, you will need upgraded Stealth and Mercy Killing in the early game. This helps pull attention away from DW Rogue. From there, follow the Evasion and Hidden Step passives. Follow this by getting Upgraded Shadow Strike – using this will place you in immediate Stealth, allowing for another attack or to reposition. The Ambush passive is not necessary at the early game but will be very valuable in the late game, as it helps with Armour Penetration.
Blackstork also suggested this. In essence, choosing Knockout Powder in the Subterfuge tree, instead of Twin Fangs. Already having Shadow Strike works better than Twin Fangs and will allow you to combo with nightmare/rupture with the proper teammates. Definitely something to consider.
Poison tree is exclusively upgraded Poison Weapons. While Looked Like It Hurt and Cheap Shot are great skills to have, it requires too much investment into your points. Blackstork suggests this strategy. Basically: Flask of Fire + Poisoned Weapons + Flank Attack = ~4 enemies Poisoned. Be aware of the time it takes to Activate each ability to get precise numbers.
Tempest requires every Flask upgraded. Follow the Fury of the Storm and Killer’s Alchemy passives. Killer’s Alchemy grants 15% DMG Bonus, which is a massive boost on all attacks. Ride the Storm is the other necessary skill, which will allow you to spend more time in your Flask’s to maximise the damage done through Activated Skills and Stealth. Flaskmaster is a useful general ability.
Tactics-wise, going for an immediate backstab is not the most optimal choice when playing Nightmare. Because of this, once you inflict Poison to do damage over 8 Seconds (which is also the amount of time it takes nearly all of your Activated DW Skills to cooldown) you can reposition in Stealth to avoid getting killed in one hit.
A basic strategy is as follows: Poison (50% of Weapon DPS) + Flask of Fire + Stealth (50% + DMG Bonus) + Flank Attack (200% DMG). This will Poison your target and put you back in Stealth. In addition, your target is poisoned. Follow this up by either activating Flank Attack again. Flask of Fire will expire but you will still be in Stealth. Activate Shadow Strike (~600% DMG w/ Stealth’s added 50% DMG Bonus) and no cooldown. Cast Flask of Lightning and use Twin Fangs.
The Passive Dance of Death makes this a viable strategy, as you can one-shot kill low-armour enemies if Poisoned Weapons was successful.
NOTE: Flask of Fire gives Focus Abilities no cooldown.