Leeching Plate is a heavy armor set. When you take damage, you have a 20% chance to summon a cloud of leeching poison under the assailant. The cloud deals 1200 Poison Damage in a 5 meter radius every 1 second for 5 seconds and heals you for 100% of the damage caused.
Leeching Plate is a good option for "tanky DPS" players who wish to balance survivability (heavy armor, health buffs) with offensive abilities (Poison Damage procs). Note: The "Grievous Leeching Ward" shield you receive as a quest reward is not only a Unique item (which means you can only have one of this item in a character's inventory or in your bank or housing storage) but also brings some "personality" to the table, which makes equipping and using it quite entertaining.
The Leeching Plate 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 heavy 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