Unlike any other power, soul power was a force that existed everywhere, yet was known by very few.
It was everywhere because all things possessed souls, whether living or dead, gods or mortal beings. Even in World of Warcraft, a world composed of the six great powers of Holy Light, Shadow, Life, Death, Arcane, and Fel, the most widespread power was still the seventh: the soul.
Souls existed within everyone, yet few truly understood them. Holy Light, Shadow, Life, Death, Arcane, and Fel all had countless users and believers. Many studied the nature and laws of these powers, allowing each of them to form a relatively complete theoretical system.
Only the soul was different. Though it was everywhere, those who truly understood it were exceedingly rare, let alone those who delved deeply into and harnessed its power—after all, everyone only had one soul. If something went wrong during reckless research, the whole person would be done for.
This directly made soul-related spells seem immensely mysterious, while attacks targeting the soul were even harder to resist.
In Silvermoon City and Dalaran, research into souls was basically taboo. Only a small number of mages were permitted to conduct related research under supervision. Cultists delved deeper into souls, but their studies were filled with superstition, chaos, and incomprehensible dark rituals, packed with conclusions that were impossible to distinguish as true or false, along with mad ravings.
Nevertheless, one thing was certain: for spellcasters native to Azeroth, there were very few ways to resist soul attacks, especially when facing an expert on the Lich King's level. As for non-spellcasters, there was even less to say—they were basically getting taken for free when faced with soul attacks 090.
It was precisely for this reason that Ner'zhul had been able to support Arthas time and again