Amber Plasm is a magicka-based light armor set. The 5-piece bonus adds 250 Magicka Recovery and 250 Stamina Recovery.
The recovery also can be increased by modifiers such as Major or Minor Intellect etc.
This is a very popular PvP set for magicka builds, as it provides you with a lot of Magicka Recovery for sustain, as well as Stamina Recovery to be able to block and dodgeroll more of the incoming attacks against you.
The Amber Plasm set is part of the Shadows of the Hist DLC. It drops in Ruins of Mazzatun in Shadowfen. Each set item is bound on pickup. You can aquire light armor, jewelry and weapons for this set in the Mazzatun motif style.