Alessian Order set is a heavy armor based set. The 5-piece bonus increases the immunity to disabling effects after using Break Free by 3 seconds.
The normal immunity lasts for 7 seconds, so another extra 3 seconds is quite a lot of extra time. This can be very helpful, especially as a tank in Cyrodiil in the PvP zone. After breaking free of a stun when wearing a full set of Alessian Order, you will get a 10-second immunity to disabling effects. You can also use a Immovability Potion after the 10 seconds to gain even more immunity to disabling effects. If you time those two mechanics correctly, you can gain immunity for a very long duration.
Alessian Order is part of the base game and can be found in Cyrodiil from the Vlastarus Elite Gear Vendor or from the Rewards of the Worthy. Each set item is bound on equip. You can aquire heavy armor, jewelry and weapons for this set. The set uses the Aldmeri Dominion, Daggerfall Covenant or Ebonheart Pact motif, depending on your alliance.