I'm surprised Blizzard has not given more attention to Kel'thuzad, when he is one of the best characters in Warcraft, and given his enormous significance in WC3. After Naxxramas 2.0, he seemingly disappeared without a trace, and it makes you question if he really just died after his second defeat. But unless some absurd retcon says otherwise, it is stated a Lich can never die until their soul shard is destroyed, after which, they will fade from existence.
This shard, seen on Kel'thuzad's first defeat, was never seen after his second defeat, and the Lich King made no effort to bring him back or mentioned him (presumably for his failure).
What happened to him?