With the old god's they've been part of the Warcraft lore for some time now. I don't know if there was anything in the first and second Warcraft games but I'm certain their presence started in wc3 for sure, in the background to everything else.
When it comes to it, while n'zoth has been regarded as the weakest of the old god's (I can't help but think they've only said this because seeing his full power unleashed gives a stark outlook on how powerful the other 3 were at full power), regardless, he has been the most influential when it's come to wow lore.
He was the one that created the emerald nightmare. He also was the one that drew in azshara and created the Naga. He joined the others in corrupting deathwing, and he was the old god that after yogg and c'thuns defeat pushed deathwing into attacking azeroth in cataclysm, as well as being the one who commanded ragnaros and al'akir still in that time.
The others might have been more powerful, but just in terms of his influence on the suffering his actions has brought to the mortal races of azeroth, his one seems the most personal. Which makes fighting him seem to be settling a score.