Sellistrix is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Sellistrix (unique) style.
The 1-piece set bonus adds 1096 Maximum Stamina.
2-piece set bonus: When you deal damage, you have a 10% chance to create an earthquake under the enemy that erupts after 1.5 seconds, dealing 5030 Physical Damage to all enemies within 4 meters and stunning them for 3 seconds. This effect can occur once every 5.5 seconds.
The Shoulder can be acquired by using an Undaunted key on Glirion the Redbeard'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 Arx Corinium which is part of the base game and located in Shadowfen. 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.