Ornaments are decorative items which you can equip in the customization menu as shown below:
https://screenshotscontent-t4001.xboxlive.com/00090000012994... For this achievement, you must equip these items on every character from a single faction. Naturally, this requires you to unlock all the heroes in one faction which I believe requires 1500 steel.
You can choose one of two ways to unlock ornaments. You can earn steel in MP matches or buy steel packs to unlock three for the knight faction (one hero does come with an ornament unlocked) or complete a chapter of the story on Realistic, as one of the rewards for finishing a faction's chapter is an ornament for each character. The latter makes more sense as you have to complete the realistic difficulty to earn the story-related achievements, so you can kill two birds with one stone.