Actually after using the ice stone. The waiths are due to using ice stone power. Not naksu sword
Using the sword means he used the power of ice stone to kill the soul shifters. He keeps naksu sword for just simple one reason is that he didn't want to move on from her. That sword was a reminder for his goal. To destroy the people who did that to his master and lover.
So, it is the ice stone not naksu's sword that cause the waith
cdrama are just meh... at the moment
BUT,
In part 2 they are GASOLINE AND FIRE.
The level of angst
Jaillll
So, it is the ice stone not naksu's sword that cause the waith