Noticed this when the season started, all classes get two ornament, and warlocks only one? but then saw it in previews and thought it's bugged. A shame, if it'll never be sold for bright dust because of this, wanted to buy it, looked like the coolest warlock ornament yet...
There is a second warlock ornament in the database, it turns the Skull of Dire Ahamkara into Riven's face, but I guess its not in the game yet. https://www.light.gg/db/items/475012491/the-want-of-lies-and-wishes/
that's the one we are talking about, according to eververse predictions it should have been available for BD this week from Tess, but there is just a blank space for warlocks... whereas for hunters and titans she sells one of the seasonal ornaments :(