In Words of Radiance,
Tarvangian tells Szeth that Kaladin must have one of the Honorblades to explain his using Stormlight.
to which Szeth responds "one of the other 7 is missing?" The problem is that nine were left after the Heralds broke the Oathpact and szeth has one, so where is the ninth?
Someone took it, obviously.
Spoilers for Oathbringer lie ahead:
The man called Darkness, who almost kills Lift in her Interlude, and revives Szeth at the end of Words of Radiance, has it.
And now the explanation, which has even more spoilers...
The Shin did have all nine Honorblades - Szeth mentions this in the same moment Darkness reveals the Honorblade to him. They have 8 now because the sword of Nale, patron of the Skybreakers, vanished. Darkness is Nale, or Nin, one of the Heralds, and he believes he simply retrieved what belonged to him.
