Aleph: How did I get here… You, do you happen to know anything?
The Demi-Fiend is using a skill called
死亡遊戯 - Game of Death (Like the Bruce Lee film that was never finished), which was localised as Deadly Fury in Nocturne.
The SMT 1 protagonist is called “The Hero”, not Kazuya or Sho or Adam or anything.
And from Famitsu:
Saviours of the Diamond Realm* Contents: Explore the Special Dungeon “Diamond Realm” with the successive generations of protagonists from the series, and battle Meta** Steven. (Includes spoilers / the difficulty level is recommended for post-game)
*Diamond Realm actually has an extra 神 kanji, but that’s apparently how it was written in SMT 1 as well when it showed up.
**Literally Superhuman, but I’ll stick with the series’ localisation of the term