Sap essence is for groups of enemies
Harness magicka- Don't let this sheild drop
Swallow soul- Light attack Swallow soul rotation until dot's need to be reapplied
Impale- Execute
Twisting path, Elemental blockade, siphoning attacks, and crippling grasp, all dots
Merciless Resolve- Use when it procs
Dark shades- Use so you can have some damage still going when you reapply dots

Gear Reccomended

Clever Alchemist
(2 items) 1206 Max Health
(3 items) 1206 Max Health
(4 items) Adds 129 Spell Damage / Weapon Damage
(5 items) When you drink a potion you feel a rush of energy, gaining 653 Weapon and Spell Damage for 15 seconds

Destructive mage
(2 items) Adds 1096 Max Magicka
(3 items) Adds 833 Spell Critical
(4 items) Adds 129 Spell Damage
(5 items) When you deal damage with a fully-charged Heavy Attack, you place a bomb on the enemy for 10 seconds. When another player hits the same enemy with a fully-charged Heavy Attack they detonate the bomb, dealing 7875 Magic Damage all enemies within 8 meters. This effect can occur once every 10 seconds.

Shroud of the lich
(2 items) Adds 129 Magicka Recovery
(3 items) Adds 1096 Max Magicka
(4 items) Adds 129 Magicka Recovery
(5 items) When you fall below 33% Magicka, increase your Magicka Recovery by 1032 for 20 seconds. This effect can occur once every 1 minute.

Vestments of the Warlock
(2 items) Adds 129 Magicka Recovery
(3 items) Adds 1096 Max Magicka
(4 items) Adds 129 Magicka Recovery
(5 items) When you fall below 33% Magicka, restore 9000 Magicka. This effect can occur once every 1 minute.

Rotation

Blockade of shock-Light attack-crippling grasp-light attack-siphoning strikes-light attack-Dark shades-Weapon swap
Twisting path-light attack-merciless resolve-light attack-swallow soul-light attack x4- Merciless resolve proc-Merciless resolve-Restart

Use sap essence only when there are more than 2-3 enemies as it is cast around you and damages them all at once. more effective that way.

Gear sets that should be used are
Julianos
(2 items) Adds 833 Spell Critical
(3 items) Adds 1096 Max Magicka
(4 items) Adds 833 Spell Critical
(5 items) Adds 299 Spell Damage.

Infaliable Mage
(2 items) Adds 688 Spell Critical
(3 items) Gain Minor Slayer at all times, increasing your damage done to Dungeon and Trial Monsters by 5%.
(4 items) Adds 129 Spell Damage
(5 items) Your fully charged heavy attacks do an additional 903 damage.
Enemies you damage with fully charged Heavy Attacks are afflicted with Minor Vulnerability for 10 seconds, increasing their damage taken by 8%.
(5 items) Adds 688 Spell Critical

and 1 piece Molag Kena
(1 items) Adds 129 Weapon and Spell Damage
(2 items) When you deal damage with 2 consecutive Light Attacks you trigger Overkill for 6 seconds, which increases your Weapon and Spell Damage by 516 but also increases the cost of your abilities by 20%

Cp is yours to mess around with but i put what is reccomended

Altered Alcast Build
https://alcasthq.com/eso-magicka-nightblade-build-pve/