Shadowrend is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Shadowrend (unique) style.
The 1-piece set bonus adds 129 Magicka Recovery.
2-piece set bonus: When you take damage, you have a 15% chance to summon a shadowy Clannfear for 15 seconds. The Clannfear's basic attacks deal 3440 Magic Damage and apply Minor Maim to any enemy hit for 10 seconds, reducing their damage done by 15%. This effect can occur once every 15 seconds.
The Shoulder can be acquired by using an Undaunted key on Maj al-Ragath's chest. The Shoulder type, weight and trait are RNG based and can be from any dungeon this NPC gives Pledges for.
The Helm can be acquired from Banished Cells I which is part of the base game and located in Auridon. The Helm is guaranteed to drop from the final boss in veteran mode only. The weight and trait are RNG based.
You can mix-and-match Monster Set shoulders and helms from different sets and add two 1-piece bonuses to your build, or wear a complete set to take advantage of the full 2-piece set bonus.