You have to fight the Cursed Coffin in that arena. However, there is an item in Fargo's Souls itself that spawns the arena anywhere (Coffin Roominator), which can be acquired by shimmering the Coffin's summoning item. However, be careful to use it somewhere out of the way, and where you can reach the entrance easily, as it overrides whatever blocks are present where it spawns, and is completely unbreakable until you defeat the boss. Another thing, I actually had issues with Remnants and my mod pack crashing because Fargo's Souls kept trying to generate the arena, but upon removing it for the world generation part and adding it back in afterwards for during game play, it worked just fine in both single player and multiplayer testing.