These all have to be equipped AT THE SAME TIME for the Achievement to register. Just wearing one piece at a time doesn't work. The entire set of 7 ornaments needs to be worn together.
This is true for all of the Hero's Forge Achievements, at least for the 5 lower level ornament sets (Ebon, Beryl, Auburn, Viridian and Violet).
The Reward is 10 Achievement points...