Race:
#1 Dunmer, 7% flame dmg + 9% magicka
or
#2 Highelf, 4% elemental dmg + 10% magicka + 9% recovery

* I made my Nightblade a vampire, because the 10% recovery from stage 2 is highly appreciated.

Mundus:
Thief mundus

Best in slot Gear:
2x Kena
5x Burning Spellweave/Scathing Mage
3x Moondancer/Infallible Aether
(4)x Moondancer/Infallible Aether Firestaff Sharpend with Spelldmg Enchant (1st bar)
1x Maelstrom Firestaff Sharpend (2nd bar)

Optional Gear:
2x Skoria/Illambris
5x Law of Julianos
3x Willpower

Skills:
For Harmode Trials use a Shield instead of Sapessence, and move siphoning attacks to your mainbar (for 8% magicka passive)

Rotation:
Pre, Merciless Resolve/Siphoning attacks
Shooting Star > Spellpower Potion > Charge Heavyattack untill Ulti lands > Twisting Path > LA > Blockade > LA > Cripple > Barswap > [LA > Force pulse / Assasins Will] > LA > Merciless Resolve > Barswap > LA > Blockade > LA > Cripple > Barswap > LA > Force pulse > LA > Assasins Will > Block > Lightattack > Merciless Resolve > Barswap > Twisting Path etc...

*Potion right after ulti for 20+ free ulti charge
*Barswap cancel Merciless Resolve, if that doesn't fit in the rotation use block to animation cancel.

PS. Kena sustain needs to be tested