Since the Colosseum content was datamined a while back it was thought it may come as part of some DLC update.
Today Fromsoftware has confirmed it’s arriving tomorrow, 7 December. The good news is this will be totally free for everyone.
Come forth, warriors, and bask in the glory of the Colosseum.
This looks to be a fun PvP addition prior to any major DLC announcement. Perhaps The Game Awards on 8 December will bring some more news on a more substantial DLC update for the future.