Divinity: Original Sin 2 isn't exactly afraid of tasking you with quests that might seem impossible to get through at first. A Fate Worse Than Death, for example, has you chasing trapped souls without giving you much more than a hint on where to find them.
Necromancer Soul Jars for this quest can be found inside the Dark Cavern, a cave the entrance to which has been hidden by magic, and therefore is not readily apparent from the get-go.
Where are the Necromancer Soul Jars in Divinity Original Sin 2?
As you're probably aware by now, Original Sin 2 has got a number of locations the entrances to which are protected by magical means. While illusions do wear off when you're close enough, this also means that you need to know what you're looking for. The map above depicts the entrance to the Dark Cavern, where Soul Jars are stashed away.
By the time you've found your way into the Dark Cavern, you shouldn't have much issue in locating the Soul Jars, but it's worth mentioning that the best thing to do after you find them is simply to destroy all but two. Namely, Gratiana and Trompdoy's Soul Jars will come in handy later on, while the rest you can safely get rid of.
On a slight tangent, if you're having trouble with resurrecting your characters, we've got a bit of a guide on that, too.