One of the KEY ways to make a story compelling is to allow villains to achieve MAJOR and PERMANENT long term goals. Things that really damage the lives of the heroes. A great way to do this is to allow villains to kill heroes. It is permanent and final. It should be 1v1 on even ground to demonstrate the villains are strong. For example, the death of Varian was utterly wasted and worthless because Gul'dan has vastly superior numbers. Varian was alone. Plus, Varian intended to die in self-sacrifice. All of this robbed the event of building up the credibility of Gul'dan. Varian's death meant NOTHING to the story which is a shame. But terrible writing is terrible writing. M'araad died in self sacrifice to superior number as well, and that ALSO robbed the event of any meaning.
So, let's review the major villains of each expansion and how many lore heroes they actually killed 1v1 in the expansion itself.
vanilla: Cthuun - 0
BC: Illidan - 0
Wrath: Arthas - 0 (this was especially embarrassing writing, as technically Arthas defeats us, then they wave a magic wand and erase it)
Cata: Deathwing - 0
MoP: Lei-Shen - 0
WoD: Archimonde - 0
Legion: Sargeras - 0 (made a pathetic flailing swipe at the Vindicaar in his cinematic just to drive home the point he can't kill anyone)
So now we come to BfA. How many lore heroes will N'Zoth kill?
Remember, off-screen doesn't count. For the kill to have emotional impact, it must be 1v1, no self sacrifice, and demonstrate the villains awesome power.