In Elden Ring, you’ll find a wide range of characters that will share their plight with you, most of these will have Quests that span multiple locations. This page will detail where you can find Blackguard Big Boggart and complete his Questline.
First Meeting - Boilprawn Shack
Your first meeting with Blackguard (as he is knwon by at first) is likely to be through the start of Rya’s Quest which is started nearby. She will send you to Blackguard asking to retrieve the Necklace that he stole from her. Blackguard can be found at Boilprawn Shack which can be found a little to the southwest of the Academy Gate Town Site of Grace in the center of the lake in Liurnia (alternatively a little to the northwest from where you spoke to Rya).
Once you reach his shack, he admits he has the Necklace and you’re presented with two options. You can either purchase it for 1000 Runes, or you can just choose to kill him, at which point you’ll be able to loot the Necklace from his body. It should go without saying but avoid killing him at all costs should you wish to progress and complete his Quest, this operates independently of Rya’s and is heavily related to Dung Eater’s Quest.
Upon agreeing to buy it, you’ll gain access to his Shop, although the only other item he offers is Boiled Prawn, a useful consumable that will boost physical negation for a short time. You’ll need to purchase at least one for him, to progress his Quest as he remarks that anyone with a taste for prawns is someone he can trust, striking up a friendship with him. This will allow you to summon him for the Magma Wyrm Makar Boss in the Ruin-Strewn Precipice dungeon.
Second Meeting - Leyndell, Capital Outskirts
Once you reach Altus Plateau, head to the northeast of the city and you’ll find yourself in the Capital Outskirts. From the first Site of Grace in the city, Capital Rampart, exit outside to the northeast and follow the path down to the water below, to the west you’ll find Blackguard has set up camp.
Speak with him and you’ll learn that he’s selling Boiled Crabs now, which are a better version of the Boiled Prawns he sold before. Exhaust his dialogue here and he’ll mention about the time he spent with Dung Eater in prison, remarking that you should steer well clear of him.
Final Meeting - Leyndell, Capital Outskirts
You must now progress Dung Eater’s Quest until he mentions that he will be waiting for you in the outer moat. Return to Blackguard and exhaust his dialogue to learn that he believes Dung Eater is somewhere nearby. Refresh the area and you’ll find a wounded Blackguard who begs for his death, oblige and you’ll be able to obtain a Seedbed Curse, Blackguard’s Bell Bearing, Iron Ball, and Blackguard’s Iron Mask. You’ll also be invaded by the Dung Eater here, defeat him and you’ll obtain the Sword of Milos and progress his Quest.