My Shady Nemesis

Just got this achievement as a 228 Survival Hunter, with a level 13 Valeera, and rank 1 Porcelain Blade tip, and Overflowing Voidspire. if you have not already, be sure to visit Valeera in Silvermoon City to take the Nulling Nulleaus quest for a free toy for killing Nullaeus, and a full 100 coffer key shards (including the ones from it's prerequisite quests).

his mechanics are:

Emptiness of the Void - kickable aoe cast that hits me for about 660k. this must ALWAYS be kicked, or if you're a class with an immunity, (Aspect of the Turtle, Ice Block, etc) you can use that instead. Valeera will kick this for you if she's set to dps or tank.

Devouring Essence - non-kickable cast that will always be cast on YOU, even if your pet has aggro. it does about 30k shadow damage every 2 secs, and lasts 18 secs. it is a dispellable magic debuff. Valeera will dispell this for you if she is set to healer.

Nullaeus will summon 3 packs of adds at certain hp intervals throughout the fight. when Nullaeus summons adds, he becomes untargetable, immune to damage, and starts casting a spell to keep him out of the fight for roughly 40 seconds, allowing you to focus on adds while he does his thing. he will re-enter combat after you either kill the adds, or he finishes his cast.

at 75% hp, Nullaeus will summon 2 adds, which have a jump spell that places a circle on the floor, and can they can also put a 16 second bleed on their target. additonally, lasers will periodically draw lines through the arena, leaving behind purple puddles that deal about 16k damage ever second you while you stand in them.

at 50% hp Nullaeus will summon a bunch of small tick adds. they're super squishy and you can aoe them down easily before they can do anything. additionally, an orb will float around the arena, which will pull you in if you get too close, and deals about 39k every second damage if you stand in it

at 25% hp Nullaues will summon 1 ogre who casts kickable shadow bolts, and a kickable Shadow Crash, which puts a circle on the floor after the cast, and a curse that slows you by 30% for 5 minutes. additionally, Nullaeus will begin stacking a 10% damage buff.

Personally, I saved Takedown for all 3 sets of adds, and was able to kill the first pack by letting my pet tank one add, and tanking the other myself, using a SoTF charge for the bleed, killed pack 2 with no issues, and used Bloodlust to burn down the final add (i found if i didn't use lust i'd need to fight Nullaeus and the add at the same time, where if i did use it, i could kill the add easier before Nullaeus could come back. may not be necessary with more gear, or on other classes).

Since Nullaeus will not attack you for a while after summoning adds, you are free to interrupt the Ogre's spells, as long as your interrupt will come back before Nullaeus begins attacking you again.

on death he dropped 35 Hero Dawncrests (in 2 stacks of 30 and 5, the stack of 30 crests didn't count towards my season maximum, however the stack of 5 did) 100 voidlight marl, 1 Nullaeus Domaneye, 1 Osseclad Waistbone (warbound, veteran, leather belt) 1 Trovehunter's Bounty, 1 Ominous Dominus (non-combat pet) and rank 2 Entropic kris for me. killing him does NOT count towards the great vault.

additionally, he appears to have some sort of treasure room in the back? but it has nothing in it, so I'm not sure why it's there. all loot was on the boss' body when he died.

I was able to do this today as a Protection Warrior ilvl 236. I nearly died a few times due to not being able to interrupt his spell in time (as more mechanics enter the fight it's sometimes hazardous to approach him as he casts it when you're being pulled away by the gravity orb or have to dodge some purple stuff on the ground), but Valeera's healing saved my bacon.

His Devouring Essence spell can be Spell Reflected, I used Endure Pain for his big strike uninterruptable attack (or Shield Wall sometimes), and his Emptiness of the Void can (and should) be interrupted every time. As a Protection Warrior, I didn't have time to completely kill the adds in the first and third intermissions but they can be cleaved along with the boss, the second adds wave was a lot easier thanks to cleave damage, many adds but little health.

Not bragging at all, but I was able to do it on my first attempt so it's probably not too difficult compared to previous Delve nemesises (?) that gave me more trouble than him.

Funny story, I was going for the pet "Ominous Dominus" that has him as a source in the pet journal and it didn't drop. I went to Wowhead and found out it actually drops from the Nullaeus cache he drops in random delves upon being defeated in his easier form. /facepalm

Good luck everyone!

Since you asked: the plural of 'nemesis' is 'nemeses'. 🌈⭐