The Venomous Revenge side quest in Hogwarts Legacy becomes available during the main quest to Prepare for Your Search for the Final Keeper main chapter after completing a couple of the main quests. Travel behind the Three Broomsticks in Hogsmeade to find a man muttering to himself. Speak with him to learn his name is Ackley Barnes and that he was cut out of his herbology trading village. He’ll then ask you to steal his ex-partner’s Venomous Tentacula. You can choose to steal the one he’s asking about or bring him another posing it as the one he asked you to steal.
If you decided to steal the plant, travel to Lawley’s Cellar between Zonko’s Joke Shop and J. Pippin’s Potions then head inside. Sneak past Mr. Lawley heading deeper into the cellar. In the next section, you’ll have to defeat some spiders that will jump you. This will clear the path leading to the Venomous Tentacula. There are also ample plants you can collect while you here. After you grab the Tentacula, return to Mr. Barnes to hand it over completing the quest. You receive 300 gold and an appearance for Dragonhide Herbologist Gloves.
If you decided to find another way, you can buy a Venomous Tentacula from Dogweed and Deathcap the deliver that to him in place of the one he asked you to steal. You’ll still receive 300 gold (500 if you asked to be paid) and the Dragonhide Herbologist Gloves. Please note that after this quest completes, you’ll still be able to go into the cellar and loot whatever you’d like!