Major spoilers for Nightflyers! Turn back now!
Why does a shard of obsidian in the heart of the Night King turn him into a cold god instead of killing him? Or protect Benjen Stark from becoming one of the mindless army of the dead? The answer comes from George R.R. Martin's short story Nightflyers and novel Dying of the Light: the whisperjewel.
Video episode: https://youtu.be/c9aNN2v5exc