The modder and data miner Lance McDonald shared the find on his YouTube channel.
With the help of a “flying” camera, he managed to examine the insides of a large location, which is not available in the game itself and cannot be accessed in an honest way.
This is a kind of Colosseum with different entry points and lifts. McDonald believes the large arena could be used in upcoming multiplayer DLCs. Similar PvP zones were previously in Dark Souls II, and in Dark Souls III the arena came along with the story addition.