Archenemy was an old Magic sub-game where one player (the Archenemy) had 40 life and special extra powerful Scheme cards they could play every turn and face-off against 3 other players. That was for 60 card formats, since they’re bringing it back with Duskmourn Commander, they’ve updated it to be 60 life for the Archenemy and a combined 60 life for the hero team

You can read more at https://magic.wizards.com/en/news/announcements/evolving-archenemy