It was absolutely N’zoth’s plan to deliver the blade to Sylvanas, knowing she would use it to make a deal with Azshara who he would have known was trying to betray him so she could take all the power for herself. He knows Azshara only wants to rule the world at the top alone, seeing nobody else as her equal. She thinks she can outsmart N’zoth, Sylvanas thinks she can use them both, and N’zoth is pulling all the strings the whole time.
N’zoth wanted Azshara to get the blade, to die to the Alliance and Horde, that’s why he tested us already and had us kill his servant so that he knew we were strong enough to defeat Azshara and stab him in the heart eventually with the very blade he made sure would stick around on Azeroth before Knaifu went off on her galactic journey where we find creatures like the blue fox/drake cash shop mounts that hint at the next expansion.
N’zoth schemes too much he would have planned the entire thing out, including his death at the hands of the players, which would allow him to continue the next phase of his plan in the Shadowlands. It all adds up.