Duelists are fighters who specialize in one-on-one combat, obviously. The Duelist's main weapon is their sword, which they use to
You thought you were going to read a blog post about fancy sword fighters, but really you've activated my trap card! Despite my never having watched Yugioh, I will make a decisive post about it because I think its hilarious.
A/Δ: A Children's Card Game, +1 Card
B: The Shadow Realm, +1 Card
C: Heart of the Cards, +1 Card
D: Play Pokemon for Everyone's Souls, +1 Card
A Children's Card Game
Get weirdly invested in some cards (and out of game bring a deck for yourself and enough for the DM to make ones for certain NPCs)
You are really good at one game (mtg, pokemon, yugioh, etc. It should be one everyone at the table knows). Pare down the game in whatever way makes it quick enough for the game-in-a-game to not be an issue. You have one card, unique in your deck, which basically makes you win if you can get it out (+1 every level). The DM might let you "summon" that card, as though casting a spell. If you challenge someone to a card duel in game, they probably know the rules, have a deck, and might accept if intelligent and not currently violent.
The Shadow Realm
If someone would otherwise accept your duel, you can set some ludicrous material or metaphysical stakes. The stakes must be in the possession of the participants, but can be things like memories, working limbs, or their corporeal form. At the end of the duel, the loser loses their stake (up to the DM if the winner gains their stake). If you send someone to the shadow realm that still counts as murder.
The Heart of the Cards
You can lose 1d10 HP, or 1 Card to search your deck for a card and put it on top.
Play Pokemon for Everyone's Souls
You can stake the fate of the world on a duel. That is, you can decide to start the apocalypse if you win the duel, or stop the apocalypse if you win against the one who started it.