The pack is a drop from Oily Invertebrate at the (Mixed Monstrosities) slime pools in Maldraxxus and although I happen to be a Necrolord it doesn't matter which Covenant you are in order to obtain the pack.
[npc=157312] is the mob you need to summon
In Maldraxxus at the slime pools (Plague Watch flight point)
/way 58.6 74.2
To summon Oily Invertebrate you need a total of 30 oils on Pool of Mixed Monstrosities in the following proportions:
10x Viscous Oil 10x Mephitic Goo 10x Miscible Ooze. (10 of each color)
Viscous Oil drop from Burning Extract and Colossal Plasma. Around /way 60,0, 76,0
Miscible Ooze drop from Diseased Remnant and Infectious Monstrosity. Around /way 57,8, 65,6
Mephitic Goo drop from Colossal Goo and Prodigious Animate. Around /way 61,2, 79,0
The turn in is to Boil Master Yetch at the pools
/way 59.12, 73.83
Slay Volatile Jelly in the House of Plagues - In Maldraxxus (SouthEast - Plague Watch flight point) appx /way 66 74
You can talk to [npc=161841] if you need protection from Caustic Fog
Slay Constructed Ooze in the Stitchyard - In Maldraxxus (North West - Spiders Watch flight point) appx /way 23 36
Around the back side of the Stitchyard
Slay Bubbling Sclerosis in the Theater of Pain - In Maldraxxus (Middle - Theater of Pain flight point) found inside the arena in the area's between the stairs
Slay Necrotic Droplet in the Temple of Courage - In Bastion (Middle West along the edge - Temple of Courage area the closest flight point is either Aspirant's Reach or Hero's Rest, neither is super close) appx /way 38 52
To the right of Necrotic Wake entrance in the area where the Bastion summoned rare is
Slay Primordial Sludge in the Blistering Bog - In Revendreth (NorthWest - Dominance Keep flight point) appx /way 33 31
On the backside of the wall
Slay Bubbleblood in the Forgotten Wounds - In Maldraxxus (middle North - Keres' Rest flight point is closest) appx /way 52 35
He is a relatively constant spawn, if he's not up just wait a little bit or look in group finder
Slay Resonant Misery in the Maw - In The Maw https://www.wowhead.com/npc=169780/resonant-misery [npc=169780]
In Gorgoa:River of Souls
Slay Congealed Ash in the Ember Ward - In Revendreath (SouthWest corner - Sanctuary of the Mad flight point) appx /way 25 55
Around the backside of the wall in a pool
Slay Bubbling Purity in The Temple of Purity - In Bastion (SouthEast along the edge - Aspirant's Rest flight point) appx /way 57 77
Near Beasts of Bastion World Quest
Slay Margrave Stradama in Plaguefall - The Dungeon, the last comment I saw said Mythic Plus did NOT count but Normal did, I did this on Normal and it counted.
While you're in Plaguefall don't forget to pet Hairball for the Nine Afterlives Achievement