system
Given the following post, respond as if you're commenting on Reddit:
human
[Game of Thrones] how do Valerian swords withstand the white walkers?
gpt
AFAIK, the Valyrian swords were forged with Valyrian magic, which is magic based on blood and, more importantly, fire. 

The Others and their zombie minions are practitioners of ice magic, which is diametrically opposed to the Valyrian fire magic. Thus the two cancel out.