A Spiritual World
The Norsemen live in two worlds: one is visible, tangible; the other is the world of spirits and Daemons, lying just beyond the senses. Though current philosophical trends of Tilea and elsewhere emphasise the empirical, that which can be studied and interacted with, the Norse believe what they see around them is the lie, a deception created to test them. Instead, the Spirit World is the truth, and only through the guidance of their mystics and the blessings of their Gods can they penetrate the veil of the senses and peer into the true reality. Since life as experienced by the senses is a deception, the Norsemen do not cling to life like other races. They throw themselves into the thick of combat to show their worth to their Gods and their ancestors, all in the hope of receiving a blessing, or to be plucked from the dream in death by one of the shadowy Warrior Hags to join their fellows in the Halls of Glory. Pain, suffering, and other physical maladies are all illusions and are accepted as part of their existence.
It's believed Norsca's proximity to the Chaos Wastes lends itself to this way of thinking. The Shadowlands are strange and ever-changing. An ordinary boulder may stay in the same place for a thousand years, and then one day pick itself up and move to another spot. Birds may fly through the air one moment, and then slither as a serpent on the ground the next. Storms come and go with no warning. The sun may rise or not, and even the very stars seem to change. The world of the north is perpetually in a state of flux, and no laws apply to this land, lending a dreamlike quality to this wild land. Hence, life and death, health and sickness are all just aspects of this great dream they collectively experience. So when a Vitki conjures up a Daemon, the Norsemen believe they are getting a glimpse of reality. And given these essences have mutable forms, it's no great logical leap to suggest the manifestation of mutations is a mark of divine favour—a blessing granted by the Gods to set the chosen apart from the mundane.