From Ymirheim Wiki

Duel is a feature that allows a player to challenge another player to a basic PvP activity.

To initiate a duel, target a player and type a /duel command in an in-game chat. The challenged player has to accept the duel request.

Duels were firstly introduced in a pre CBT2 progress report post.

The project Ymirheim developers' comment regarding the current state of the newly implemented feature:

"For now, the duel implementation is very basic: there is no win condition, no restrictions… It just lets two players to fight each other. Everything else is up to you."