Duel arena (obstacles). You'll need to dig at the location shown in the map below while having the clue scroll and navigation tools in your inventory. You can only go inside the arena while dueling with another player, usually there is a player around willing to help you. Have the obstacles option checked when challenging your opponent or you will end up in the wrong arena.

Required items: clue scroll, watch, sextant, chart, spade, armour, weapon and food

Possible way of transport: Ring of dueling.