Sheer Venom is a medium armor set. When you deal damage with an Execute ability, you infect the enemy, dealing 8428 Poison Damage over 6 seconds. This effect can occur once ever 6 seconds.
The Sheer Venom set is part of the Imperial City DLC. It drops in Imperial City Prison in the Imperial City. Each set item is bound on pickup. You can acquire medium armor, weapons and jewelry of this set in the Xivkyn motif style.
If you need more information about the Imperial City, make sure to check out the full Imperial City Guide