Well, I just read up on the shadow realm. I plan to reveal the whole plot pretty soon, so the duels will begin to happen in the shadow realm in a little bit, probably around/after Taerund's duel, which I plan to be an Action Duel.

At this point in time, the closest thing to the shadow realm will be the "Shadow Gate" field spell (The effect-free field spell that I have designated for Action duels)

Later on, around the duel with the Arbitrator, we will have a duelist kingdom style Shadow realm duel (Just the two duelists in an area of darkness)