It is a Daedric Artefacts, reward from the quest Beothiah's Calling. You get it after killing the former Beothiah champion, therefore becoming the new champion.
The Ebony mail is the sign that you are the actual Beothiah Champion.
Great for sneaky type. It is the ONLY piece of armor not being boots that have Muffle on it. Also, it has a permanent Poison Cloak, activate when sneaking, neat for melee.
But be careful, the Poison cloak can make you detected if you intend to sneak kill your target with your dagger, as it will damage you target BEFORE you can get close enough to sneak attack him.