YOU MUST HAVE STARTED THE GAME ON CORE DIFFICULTY AND REACHED THIS POINT WITHOUT CHANGING IT.First: You need to actually spawn the thing in which is another problem entirely. To do it, go to the market square before the defender's heart attack, go to the top right corner where you find the guards being attacked by the Nabasu, see the cutscene, go towards the top right corner where you face ghouls and shadow creatures, be careful with their ability damage, go towards the end until you find the gnome with a taste for necromancy, kill him and clear that section.
This gnome can be difficulty because 3 ghouls, 4 cultists and he casting haste and blindness can be complicated, this fight can be nerfed with a well positioned selective grease.
Second: Do the Defender's Heart defense and go back to where the gnome was and you will find the nabasu once you get close. Buy death ward scrolls and freedom of movement scrolls from the cleric in defender's heart. A Nabasu has that level-stealing gaze and this one always starts the combat trying to mass hold person, counting with this game balance system, you have a 1% chance of overcoming it naturally with all characters so better to invest in some scrolls.
Other than that, buff yourself the best you can and debuff him too and you should be able to get the kill.
Credit to ALEC for their
guide.