Taken from Tober's Steam guide.

If you venture into the ember veins, you will encounter Essence Drinkers, Essence Gorgers, and the like. These ashen grublike creatures can drop adornments, which are a sort of mystical addition to your regular gear, and which grant additional stats while you are inside an ember vein. Group side adornments are Mythical (violet) tier, while solo side adornments are Superior (cyan) tier. The adornments comprise two sets, which grant additional bonuses the more pieces you have. Note that both Mythical and Superior adornments count toward each set bonus.

In an ember vein's group loop, you will find a side room containing a well of ember essence in the floor, at which a large number of ashen creatures gather. Among them will be an *Essence Drinker or an Essence Gorger. *Essence Drinkers have a small chance of dropping a Mythical adornment, while Essence Gorgers are guaranteed to drop one. Gorgers are strong (4 chevrons), and summon adds to help them. Be sure to kill any Essence Bursters that appear right away! They will explode if you don't.

It's a good idea to bring some ember resist gear and potions when you go to hunt for your Superior adorments. Also, by the way, an essence well is a great place to fill your ember stone and to level up your alchemy.

In the solo loops, you will occasionally run into a small wandering Essence Drinker, which might drop a Superior tier (cyan) adornment. These have about half the stats of the Mythical adornments.

Some adornments come in three varieties, originally intended for strikers, defenders, and supporters (for group loop adornments, Essence Challengers, Guardians, and Wielders, respectively, and for solo loop, Essence Fighters, Protectors, and Workers). The devs removed the role restrictions shortly after introducing them, but kept the variety, so you have some choice in how you build your character. A few of the adornments only come in one variety and are called Essence Soaked or Essence Dipped.

This guide contains screenshots of the group and solo side adornments I have come upon, or that have been shared with me. Thanks to Andross, Feedpan, Genrah, Jericco, Kisulae, Littlezoe, Mandingo, Raxtin, Roscoejr, Zoonzerker, and (Anonymous) for sharing their adornments info. Special thanks to Kiriko for much updated info.