I’m curious to know what he did to make Diablo banish him into the void. Na-Krul only appears in D1 Hellfire. Maybe Mephisto and Baal wanted to create a “new brother” and he was a “failed prototype” before Diablo?
Diablo: Hellfire is non-canon. It wasn’t even made by Blizzard North, but by Sierra, a separate (and now defunct) studio. It’s some interesting reading regarding the relationship between Vivendi, Blizzard, and Sierra.
Blizzard North objected strenuously to the Diablo: Hellfire expansion, but at the time, Blizzard’s parent company, Davidson & Associates, signed off on it regardless.