Velidreth is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Velidreth (unique) style.

The 1-piece set bonus adds 129 Weapon Damage.

2-piece set bonus: When you deal damage, you have a 20% chance to spawn 3 disease spores in front of you after 1 second that deal 10320 Disease Damage to the first enemy hit. This effect can occur every 9 seconds.

The Shoulder can be acquired by using an Undaunted key on Urgalag Chief-Bane'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 Cradle of Shadows which is part of the Shadows of the Hist DLC 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.