Wielding Greatswords in Elden Ring is a brilliant choice, as you can customize your build around them. The Marais Executioner's Sword is one of the most powerful Greatswords in the game: Here's how to get and use this weapon.
ON MARAIS EXECUTIONER'S SWORD
The Marais Exectioner's Sword can be obtained after defeating Elemer of the Briar in the Shaded Castle. This area is located just between Mount Gelmir and Altus Plateau, so you'll need to progress through the main storyline to reach the location. A Poison Swamp surrounds the Shaded Castle, so I highly recommend using Torrent to navigate the area.
The first Site of Grace you unlock is the Shaded Castle Ramparts, located on the southeastern corner of the large structure. Use Torrent to jump over the broken walls to access the Shaded Castle Ramparts Site of Grace. After resting at this location, make your way through the swampy undergrowth to the castle's interior.
The Shaded Castle Inner Gate Site of Grace is the closest fast travel point for the boss fight. The Shaded Castle is also home to certain key items such as the Valkyrie's Prosthesis which is required for Millicent's questline. My recommendation would be to explore the Inner Gate Site of Grace first before exploring other parts of the castle for secret loot.
