The trading leech card is one of the Gorajo cards a player can be given by the Gorajo hoardstalker found in the resource dungeon in Meilyr Clan district in Prifddinas, requiring level 95 Dungeoneering to enter. You can get one card per day from the hoardstalker, resetting at 00:00 UTC. This version of the card affects only the player who activated it, and there is a team version of the card that affects all of the members of a dungeon party. You can only have one solo card and one team card (of any type) at once.
To use this card, start a dungeon at Daemonheim, and click to activate it, consuming it. It gives a variety of extra items to the player who activated it at the start of the dungeon. It will always give a correctly-tiered piece of equipment for an empty slot, and 2-8 other items, including food, runes, potions, and even a blood necklace and hexhunter bow.